Does the combined application of organic and mineral nutrient sources influence maize productivity? A meta-analysis
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- Chivenge, P., Vanlauwe, B. & Six, J. Plant Soil (2011) 342: 1. doi:10.1007/s11104-010-0626-5
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The combined application of organic resources (ORs) and mineral fertilizers is increasingly gaining recognition as a viable approach to address soil fertility decline in sub-Saharan Africa (SSA). We conducted a meta-analysis to provide a comprehensive and quantitative synthesis of conditions under which ORs, N fertilizers, and combined ORs with N fertilizers positively or negatively influence Zea mays (maize) yields, agronomic N use efficiency and soil organic C (SOC) in SSA. Four OR quality classes were assessed; classes I (high quality) and II (intermediate quality) had >2.5% N while classes III (intermediate quality) and IV (low quality) had <2.5% N and classes I and III had <4% polyphenol and <15% lignin. On the average, yield responses over the control were 60%, 84% and 114% following the addition of ORs, N fertilizers and ORs + N fertilizers, respectively. There was a general increase in yield responses with increasing OR quality and OR-N quantity, both when ORs were added alone or with N fertilizers. Surprisingly, greater OR residual effects were observed with high quality ORs and declined with decreasing OR quality. The greater yield responses with ORs + N fertilizers than either resource alone were mostly due to extra N added and not improved N utilization efficiency because negative interactive effects were, most often, observed when combining ORs with N fertilizers. Additionally, their agronomic N use efficiency was not different from sole added ORs but lower than N fertilizers added alone. Nevertheless, positive interactive effects were observed in sandy soils with low quality ORs whereas agronomic use efficiency was greater when smaller quantities of N were added in all soils. Compared to sole added ORs, yield responses for the combined treatment increased with decreasing OR quality and greater yield increases were observed in sandy (68%) than clayey soils (25%). While ORs and ORs + N fertilizer additions increased SOC by at least 12% compared to the control, N fertilizer additions were not different from control suggesting that ORs are needed to increase SOC. Thus, the addition of ORs will likely improve nutrient storage while crop yields are increased and more so for high quality ORs. Furthermore, interactive effects are seldom occurring, but agronomic N use efficiency of ORs + N fertilizers were greater with low quantities of N added, offering potential for increasing crop productivity.
KeywordsOrganic resource quality Interactive effects Integrated soil fertility management Yield response Meta-analysis Agronomic N use efficiency N fertilizer
Soil organic carbon
Mean annual precipitation
Enhanced food crop production in sub-Saharan Africa (SSA) is critically dependent on external nutrient inputs, especially N and P. This is mainly because of negative soil nutrient balances caused by continuous cultivation with little or no addition of nutrients (Cobo et al. 2010; Sanchez 2002; Smaling et al. 1997). While mineral fertilizers are widely used globally to overcome nutrient deficiencies, their use remains very low in SSA with average application rates of 8 kg ha−1 yr−1 (Crawford and Jayne 2010; Morris et al. 2007; Smaling 2006). Fertilizer use has been limited mainly because of low availability and lack of purchasing capacity by the smallholder farmers in SSA (Morris et al. 2007). Ironically, in SSA, where more than half the population lives on less than US $1 day−1, fertilizer costs are two to eight times more expensive than in the rest of the world (Bationo et al. 2006).
Organic resources (ORs), ranging from animal manures, household composts, crop residues, leguminous cover crops, to leguminous and non-leguminous trees and shrubs, are often used as major nutrient sources to crops. However, their use in most African cropping systems is usually limited by low availability (Giller et al. 1998; Mugwira and Murwira 1997; Rufino et al. 2010). Application of ORs usually leads to increased crop yields (Kimetu et al. 2004; Mugwira 1984; Vanlauwe et al. 2001b) but depressed yields with OR use have also been reported (Mugwira and Murwira 1997; Nhamo 2002). The differential yield responses following OR application have been attributed mainly to differences in OR quality and soil fertility status (Giller and Cadisch 1995; Koné et al. 2008; Palm et al. 2001a). Quality of ORs is often defined using their N, lignin and polyphenol concentrations (Constantinides and Fownes 1994; Melillo et al. 1982). Based on the same parameters, Palm et al. (2001a) proposed four OR quality classes and developed a decision support tool for the management of OR N. They proposed to directly apply high quality ORs which have a fast nutrient release, class I (>2.5% N, <4% polyphenol, <15% lignin) and to surface apply low quality ORs, class IV (<2.5% N, >15% lignin) for erosion control. Class IV ORs induce N immobilization that can last for extended periods of time. On the other hand, intermediate quality ORs, i.e. class II (>2.5% N, >4% polyphenol or >15% lignin) and class III ORs (<2.5% N, <4% polyphenol, and <15% lignin), are to be added in combination with N fertilizers to alleviate a slow nutrient release due to biochemical recalcitrance for class II ORs and low N content for class III ORs.
The combined application of ORs and fertilizers is increasingly gaining recognition as one of the appropriate ways of addressing soil fertility depletion, especially in low-external input systems in SSA and forms an integral part of integrated soil fertility management (Vanlauwe et al. 2010a, 2002a). Greater yield benefits can be achieved following the combined application of ORs and fertilizers compared to either resource applied alone (Mucheru-Muna et al. 2007; Nziguheba et al. 2002; Vanlauwe et al. 2002a). Nutrient cycling and the associated yield benefits derived from combining ORs and fertilizers are dependent on a number of factors including climate, bio-physico-chemical soil environment and OR quality and there are intricate interactions among these factors (Chivenge et al. 2009; Giller and Cadisch 1995; Palm et al. 2001a). For example, Tian et al. (2007) observed increases in rate of decomposition and nutrient release with increase in OR quality in wetter climates but in drier climates decomposition and nutrient release was faster with low quality ORs than high quality ORs. Low quality ORs have a mulching effect that influence soil microclimate and thus enhance their decomposition (Lavelle et al. 1993; Tian et al. 2007, 1993).
While the combined addition of intermediate quality ORs with N fertilizers or the sole addition of high quality ORs can enhance nutrient cycling and increase crop yields, their effects on soil organic C (SOC) build-up may be negative. The addition of fertilizers with intermediate quality ORs may increase OR decomposition (Sakala et al. 2000; Zingore et al. 2003) and thus may result in reduced SOC stabilization compared to OR added alone. The addition of intermediate and low quality ORs may result in greater SOC concentrations than high quality ORs (Bossuyt et al. 2001; Six et al. 2001). However, recent studies have shown no long-term effects of OR quality on SOC dynamics (Chivenge 2008; Gentile et al. 2010). Nevertheless, there may be interactions with other factors such as soil texture and rainfall (Chivenge 2008; Feller and Beare 1997).
Although narrative reviews provide useful summaries of the knowledge of a given discipline, meta-analysis offers a more precise and quantitative synthesis of treatment effects by statistically comparing results from multiple studies (Gurevitch and Hedges 1999; Rosenburg et al. 2000). This approach has been borrowed primarily from medical, physical and behavioral sciences (Gurevitch and Hedges 1999) and has been used recently for ecological studies (de Graaff et al. 2006; Gurevitch et al. 2000; Knorr et al. 2005; van Groenigen et al. 2006) and agricultural experiments (Miguez and Bollero 2005; Sileshi et al. 2008; Tirol-Padre and Ladha 2006). Meta-analyses methods provide a robust synthesis of results from independent studies in a manner that is both objective and statistically defensible (Ainsworth et al. 2007; Hungate et al. 2009). Meta-analytic procedures were used in the current study to analyze the effects of the addition of ORs, N fertilizers and ORs in combination with N fertilizers on Zea mays (maize) productivity and SOC and interactions among different factors. Although publication and research bias cannot be ruled out, we believe that the studies included in this meta-analysis were sufficient to capture the diversity of soils, climate and OR quality classes that are generally used by smallholder farmers in SSA.
maize yield and SOC responses to the application of ORs, N fertilizers, and ORs + N fertilizers,
the influence of OR quality, both when applied alone or in combination with N fertilizers, on maize yield and SOC responses,
effects of OR-N and fertilizer-N quantities on maize yield and SOC responses,
the interactive effects of combining ORs with N fertilizers and how these could be influenced by OR quality and OR-N quantities,
the interactions of the above with soil texture and mean annual precipitation (MAP).
Materials and methods
We identified 57 studies carried out in smallholder farms and experimental stations under rain-fed field conditions in SSA where ORs and N fertilizers were added separately and in combination with each other (See Appendix 1). To be included in our meta-analysis, the studies should have reported maize yields following the combined addition of ORs with N fertilizers, sole ORs, and/or sole N fertilizers. Data from the same experiment but reported in more than one publication were not repeated, the publication with the most complete dataset was used. Twelve of the 57 studies also reported measurements of SOC. Published data reported in tables were taken directly from the publications while results presented in graphs were digitized and measured to estimate the values. Sixteen studies were carried out over a single season while only nine studies were carried out over at least 5 years. Twenty-three studies tested the repeated application of ORs over at least two seasons while five tested the residual effects of the ORs. Residual effects were estimated based on observations made in the second season when ORs were only applied in the preceding season and crops planted in both the first and second season. Crop residues were removed from the field for other uses such as fodder or were grazed in situ, except in cases where they were used as the OR treatment.
Categorical variables used in describing the experimental conditions
Sand (<20% clay)
Loam (20–32% clay)
Clay (>32% clay)
Mean annual precipitation
Organic resource classa
Class I >2.5% N, <4% polyphenol, <15% lignin
Class II >2.5% N, >4% polyphenol or >15% lignin
Class III <2.5% N, <4% polyphenol, <15% lignin
Class IV <2.5% N, >4% polyphenol or >15% lignin
Organic resource N
≤30 kg N ha−1
30–100 kg N ha−1
>100 kg N ha−1
≤30 kg N ha−1
30–100 kg N ha−1
>100 kg N ha−1
Summary of number of studies and data points of the observations made under different categories
Organic resource quality classa
Organic resource N (kg N ha−1)
Fertilizer N (kg N ha−1)
Number of data points (number of studies)
Mean annual precipitation (mm)
An unweighted effect size was also calculated and used for meta-analysis for SOC changes because some studies did not report standard deviations. In addition, since there were few studies that reported SOC measurements, it was essential not to omit studies and maintain a larger sample size. Effect sizes of ORs and ORs + N fertilizers over the no input control were calculated as the natural log of the response ratio of ORs or ORs + N fertilizers over the control. The initial C contents were not always available and hence SOC changes over time were not calculated.
Because there are many interactions among the factors that influence maize yield and SOC responses, agronomic N use efficiencies and interactive effects, there are many ways that the data could have been presented. However, we looked at the possible interactions and selected only the data that showed the most interesting trends.
Organic resource alone
Overall, yield responses tended to increase with increasing OR-N quantity but distinct differences were only observed in clayey soils (Fig. 4b). Across all soil textures, mean yield responses were 100% in experiments where >100 kg OR-N ha−1 was added, whereas in experiments where <30 kg OR-N ha−1 was added, yield responses were only 8% (Fig. 4b). Residual effects of ORs applied in one season were positive in the subsequent season with crop yield responses of 38% over the no input control when all textures were combined but in sandy soils there were slightly negative but not significant (Fig. 4b). In clayey soils, residual effects of ORs were greater (49%) than where ≤30 kg OR-N ha−1 (15%) was added but less than where greater OR-N quantities were added (Fig. 4b). Surprisingly, in the loamy soils, residual effects of ORs resulted in the greatest yield responses (69%) which were, however, not significantly different from when >100 kg OR-N ha−1 was added (Fig. 4b).
Organic resources with N fertilizers
Residual effects of ORs
Efficacy of ORs + N fertilizers over sole ORs or N fertilizers
Interactive effects of the combined application of ORs and N fertilizer on maize yields
Agronomic N use efficiency
Soil organic carbon
Maize yield responses
Benefits of external nutrients
Results from our meta-analysis clearly highlight the positive maize yield benefits realized following the external addition of nutrients in SSA soils in the following decreasing order; ORs + N fertilizers > N fertilizers > ORs (Fig. 2a). While the addition of N fertilizers has been shown to result in greater crop yields than ORs (Baggs et al. 2000; Bremer and van Kessel 1992; Ladd and Amato 1986), greater crop yields have been observed following the combined application of ORs with N fertilizers (Kimani et al. 2007; Kramer et al. 2002; Mtambanengwe et al. 2006). The greater yield benefits with the combined treatment have been mainly attributed to the direct interactions between the two resources where temporary immobilization of N from fertilizers by ORs may result in improved synchrony between supply and demand of nutrients (Myers et al. 1994; Palm et al. 2001a; Vanlauwe et al. 2001c). This improved synchrony enhances the use efficiency of the two resources, often leading to positive interactive effects on yield, i.e. yields greater than the sum of yields obtained following the sole application of either resource (Vanlauwe et al. 2001a). The positive interactive effects might also be due to the alleviation of other growth limiting factors such as micronutrients (Palm et al. 1997; Zingore et al. 2008). However, in our analysis across all studies, the interactive effects of combining the two resources were, most often, negative (Figs. 8 and 9). Moreover, the agronomic N use efficiency following the combined addition of ORs and N fertilizers was not different from sole applied ORs but was instead lower than sole applied N fertilizers (Fig. 10a). This indicates that the extra yields observed with the combined treatment were not caused by improved efficiency of utilization when the two resources are added together, but likely due to the extra N supplied when the two resources were added together. However, excess amounts of N were added in the combined treatment where at least 100 kg N ha−1 was added in the combined treatment in more than 70% of the studies (data not shown) with sum N added as high as 667 kg N ha−1. This could have reduced the agronomic N use efficiency (Cassman et al. 2002) and masked the possible positive interactions. Nonetheless, it should be noted that yield responses were variable under different conditions; for example different OR qualities led to different interactive effects in different textured soils while low quantities of N added resulted in greater agronomic N use efficiencies (see sections below).
While results from our meta-analysis imply no improvement in agronomic N use efficiency following the combined addition of ORs with N fertilizers compared to sole applied ORs or N fertilizers, there is a possible shift towards increased N utilization efficiency of the two resources in the long-term. Previous studies have shown lower recoveries of OR applied N in the first year of application compared to N fertilizers but with greater residual benefits than N fertilizers in subsequent seasons (Bosshard et al. 2009; Cadisch et al. 1998; Handayanto et al. 1997). We also observed residual OR benefits on average crop yield response of 40% for sole applied ORs compared to the control, implying possible build-up of nutrients in soil following the application of ORs (Fig. 6a). Moreover, the addition of ORs, alone or in combination with N fertilizers resulted in greater SOC increases compared to the control whereas SOC following N fertilizer additions was not different from the control (Fig. 12a). Greater sustainability and soil organic matter build-up in the long-term following the addition of ORs, alone and in combination with N fertilizers (Bhattacharyya et al. 2007; Bhattacharyya et al. 2008; Bi et al. 2009) also imply greater crop yields may be achieved by these treatments than sole applied N fertilizers over time. Most of the studies (48) included in our meta-analysis were carried out over less than 5 yr, with 16 of them being carried out over one season. Thus, although there were lower agronomic N use efficiencies and negative interactive effects following the combined addition of ORs with N fertilizers in the short-term, there are possible improvements on these in the long-term. This brings a need to invest in long-term evaluations of the combined addition of ORs with N fertilizers in SSA.
Organic resource quality and OR-N quantity influences
Generally, greater OR-N quantities were added with high quality ORs than low quality ORs thus the greater yield responses with high quality ORs were likely due to greater amounts of N added (Figs 4). In support, several studies have observed greater crop yields with high than low rates of OR application (Chivenge et al. 2009; Mtambanengwe and Mapfumo 2006). Similarly, greater yields have been observed with high than low quality ORs (Mtambanengwe et al. 2006; Murwira et al. 2002; Teklay et al. 2006; Vanlauwe et al. 2001c). The addition of high quality ORs, class I, results in a fast release of nutrients, which may be taken up by plants if they are in synchrony with crop demands (Kimetu et al. 2004; Mafongoya et al. 1998a; Palm et al. 2001b). In contrast, the addition of intermediate to low quality ORs, classes III and IV ORs, may cause N immobilization and, therefore, lead to lower yields than the no input control (Kapkiyai et al. 1999; Mugwira and Murwira 1997; Sakala et al. 2000). In our meta-analysis, although low quality ORs, class IV, resulted in lower crop yield responses than other OR quality classes, the decline in crop yields with class IV ORs compared to the control was only observed in sandy soils (Fig. 4a), probably reflecting the inherent infertility of sandy soils (Bationo et al. 2007; Grant 1981). This was supported by the low yields of the control and the greater yield responses following the external addition of nutrients in sandy soils compared to other textures (Fig. 1). Although the addition of low quality ORs would have been anticipated to increase moisture retention and availability (Bationo et al. 2007; Bauer and Black 1992; Tian et al. 2007; Vanlauwe et al. 2002a), nutrient limitations may have been more critical for crop growth (Greenwood et al. 1996; Ouédraogo et al. 2006) and probably more so in sandy soils where the nutrient base is low. Nonetheless, differences among OR quality and OR-N quantity classes were more distinct in the clayey soils (Fig. 4b) and this was probably because of the greater contact between OR and clay particles (Gentile et al. 2008; Mary et al. 1996). Clay particles have a large surface area, enhancing microbial contact with ORs whereas the limited contact in coarse textured soils may limit OR decomposition (Strong et al. 1999). Additionally, coarse textured soils have a lower moisture holding capacity which may also limit decomposition of ORs (Manning et al. 2008), and thus result in less distinct OR quality differences compared to fine textured soils. Moreover, soil aggregate and SOC dynamics in clayey soils are influenced by OR quality and in turn influence OR decomposition and nutrient dynamics whereas in coarser textured soils there are fewer aggregates (Bossuyt et al. 2001; Six et al. 2001).
Although OR quality was clearly important in influencing maize yield, there were generally no differences in maize yield responses, agronomic N use efficiency and interactive effects between OR classes I and II when applied alone or in combination with N fertilizers in all soils (Figs. 4, 5, 6, 9 and 10). This suggests that polyphenol content, which separates the two classes (Mafongoya et al. 1998a; Oglesby and Fownes 1992; Palm et al. 2001a), may not play a significant role in nutrient release under field conditions. While polyphenols have been shown to be biochemically recalcitrant and, therefore reduce N mineralization (Heal et al. 1997; Mafongoya et al. 1998b; Palm and Sanchez 1991), Vanlauwe et al. (2001b) showed that polyphenol contents influenced maize N uptake in a pot trial, but its impact was minimal under field conditions. Similarly, N mineralization (Basamba et al. 2007; Gentile et al. 2008; Teklay and Malmer 2004) and crop yields (Chivenge et al. 2009; Kimetu et al. 2004; Shisanya et al. 2009) have been observed to be similar for the two OR classes, probably because in some instances the polyphenols are leached out of the soil (Vanlauwe et al. 2002b). Additionally, recent studies have shown that biochemical recalcitrance may not be as long lasting as initially thought (Gentile et al. 2008; Grandy and Neff 2008; Marschner et al. 2008) but that OR decomposition is primarily driven by OR-N content (Parton et al. 2007). Thus, contrary to the decision support system proposed by Palm et al. (2001a, 1997) where they separated classes I and II ORs, we conclude that there are no differences in maize yield responses between the two classes. The current meta-analysis suggests three distinct OR classes with classes I and II falling in the high quality class, classes III and IV in the intermediate and low quality classes, respectively (Figs. 4a, 5a and 9a). Similarly, Vanlauwe et al. (2005b) showed three distinct quality classes based on short-term N mineralization assays.
When the ORs + N fertilizer treatment was compared to sole ORs, greater yield responses were observed with low quality ORs than high quality ORs (Fig. 7a). This was probably because greater yields were observed with sole high quality ORs (Fig. 4a) such that the supplementary addition of N fertilizers resulted in a small increase in maize yields. On the contrary, low quality ORs may have induced N limitations that were alleviated by the addition of fertilizer-N resulting in greater yield increases compared to sole ORs. This alleviation was more pronounced in sandy soils where the addition of N fertilizers to class IV ORs resulted in maize yield responses of 249% when compared to sole ORs, whereas in clayey soils it was only 35% with the same OR class (Fig. 7a). Several studies have also observed greater increases with the combined application of low than high quality OR over sole OR (Friesen et al. 2002; Teklay et al. 2006; Vanlauwe et al. 2002a). Similarly, Mtambanengwe et al. (2006) observed a mere 13% yield increase when class I OR, C. juncea, was added in combination with N fertilizer compared to sole C. juncea in a sandy soil. In contrast, there was a 325% and 800% yield increase with the combined addition of class III and IV ORs with N fertilizers compared to sole ORs, respectively.
Positive interactive effects have been observed when intermediate and low quality ORs are applied in combination with N fertilizers but not with high quality ORs (Vanlauwe et al. 2002a, 2005a). Thus, we expected positive interactive effects with class III and IV ORs but these were only observed with class IV ORs and only in sandy soils (Fig. 9a). However, when class IV ORs + N fertilizers were compared to sole N fertilizers, there were no yield responses for all soil textures (Fig. 7b). In addition, agronomic N use efficiency for the combined addition of class IV ORs with N fertilizers was less than 5 compared to 12 kg grain increase kg−1 N added with class III ORs in sandy soils (Fig. 10). Thus, in support of the decision support system (Palm et al. 1997; 2001a), where fertilizers are available, there are no added benefits of combining their application with class IV ORs.
In contrast to comparisons with sole ORs, when the combined treatment was compared to sole N fertilizers, yield responses increased with increasing OR quality (Fig. 7b). While there were no differences between OR classes I and II in sandy and loamy soils, in clayey soils class II ORs + N fertilizers resulted in a yield response of 25% over N fertilizers compared to 11% with class I ORs (Fig. 7b). This suggests that the combined addition of N fertilizers with class II ORs may be more beneficial than with class I ORs, as proposed by Palm et al. (2001a; 1997) but only in clayey soils. However, the interactive effects of combining class II ORs with N fertilizers were generally negative and not different from class I ORs (Fig. 9a). Additionally, the agronomic N use efficiencies were not different for the two classes in clayey soils but were greater with class I ORs than class II ORs in sandy soils (Fig. 10).
Residual effects of ORs were also influenced by OR quality with greater residual effects observed with high quality ORs, both when ORs were applied alone or in combination with N fertilizers (Fig. 6). This contradicts the general consensus that low quality ORs may have greater residual benefits due to the slow decomposition of ORs making nutrients available over longer periods of time (Mafongoya et al. 1998a, 1997; Palm et al. 2001b; Tian et al. 2007). However, given that OR-N added by the low quality ORs was less than that added by high quality ORs, it is possible that there were just not enough nutrients to be used by the following crop. The lack of residual effects in sandy soils indicates the need to continuously add nutrients to these inherently infertile soils (Bationo et al. 2007). However, while there were no residual effects of ORs in sandy soils, the addition of N fertilizers in the residual year resulted in crop yield responses of about 80% over the control whereas in clayey soils residual effects were 49% and 63% over the control for ORs and ORs + N fertilizers. These results stress the need to continuously add nutrients in the inherently infertile sandy soils and suggest that the addition of ORs in sandy soils may lead to improved water use efficiency once nutrients are added.
Fertilizer-N and OR-N quantities and soil fertility influences
The greater yield responses to N fertilizer additions observed in experiments where ≤30 kg N ha−1 was added (Fig. 3a) were likely because most of the studies with this low fertilizer-N addition rate were in areas receiving ≤600 mm rainfall annually. As shown in the analyses, greater yield responses were observed in studies done in low rainfall areas (Fig. 3a). When considering the yield differences, it was clear that the absolute yield increases due to addition of ≤30 kg N ha−1 were lowest among the N application classes (Fig. 3b). Furthermore, absolute yield increases were lowest in climates receiving ≤600 mm MAP (Fig. 3b). Thus, similar to previous observations (Vanlauwe et al. 2001c), proportional yield responses to nutrient additions (ORs, N fertilizers, ORs + N fertilizers) were greater when the initial control yields were low and vice versa (Fig. 1).
The efficiency of utilization of applied N fertilizers was greater when lower quantities of N fertilizers were applied and decreased with increasing fertilizer-N quantities (Fig. 11b). Lower agronomic N use efficiencies have been observed with greater quantities of fertilizer-N added (Cassman et al. 2002; Ortiz-Monasterio et al. 1997; Raun et al. 2002). Thus, the lower agronomic N use efficiencies observed with the combined addition of ORs + N fertilizers than sole N fertilizers were likely because of greater quantities of N that were generally added with the combined treatment compared to sole N fertilizers. However, the same was not observed when the combined treatment was compared to sole ORs where generally lower quantities of N were added compared to the combined treatment (Fig. 10a). The lack of differences in agronomic N use efficiencies between sole applied ORs and the combined treatment imply that ORs may reduce the efficiency of N fertilizers, similar to observations by Takahashi et al. (2007) where the agronomic N use efficiencies of N fertilizers were reduced when applied with compost. Although reduced N losses and improved N synchrony have been proposed to lead to increased N use efficiencies following the combined application of ORs + N fertilizers (Giller 2002; Palm et al. 2001a; Vanlauwe et al. 2002a), in the current meta-analysis there was no evidence of increased N use efficiencies when ORs were combined with N fertilizers. It is worth noting that, agronomic N use efficiencies were greater where lower amounts of N were added, both as OR-N and fertilizer-N, but were greater for low fertilizer-N than OR-N. This was particularly more so in clayey soils where agronomic N use efficiency was 27 and 37 kg grain increase kg−1 N added when ≤30 kg OR-N ha−1 was added as OR-N and fertilizer N, respectively (Fig. 11a and b). Thus, while low quantities of N as OR-N and fertilizer-N enhance utilization efficiency of added N, clayey soils tended to also promote greater efficiency of N utilization. Clayey soils are generally more fertile with greater SOC than coarser textured soils, influencing the efficiency of N utilization. In support, greater agronomic N use efficiencies have been observed in fertile soils close the homestead than those that are further from the homestead and less fertile (Vanlauwe et al. 2010b, 2006).
Soil organic carbon responses
The increases in SOC with ORs and ORs + N fertilizers and not with sole N fertilizers show the need for OR additions to increase SOC (Fig. 12a). Similar to yield responses, greater differences in SOC were observed in sandy soils than in clayey and loamy soils (Fig. 12c). This was most likely because of the low starting SOC contents such that the addition of ORs caused greater proportional increases in SOC, which, in absolute terms may be lower than in finer textured soils. In addition, because of the lower protection of added ORs and faster loss of nutrients in sandy soils than clayey soils, there is a need for continuous addition of C in sandy soils (Mapfumo et al. 2007). Chivenge et al. (2007) observed greater responses in SOC contents following the addition of mulch in a sandy soil while there were insignificant responses in a clayey soil. Although there were greater yield increases with OR + N fertilizer compared to sole OR (Fig. 2a), there were greater C contents with sole OR than OR + N fertilizer (Fig. 12). This is likely because the added N fertilizers enhanced decomposition of the added OR, which likely resulted in greater N supply but also greater losses of added C (Khan et al. 2007; Nardi et al. 2004). In contrast, Bhattacharyya et al. (2007) observed greater SOC with ORs + N fertilizers than ORs, whereas Nayak et al. (2009) found no differences in SOC between sole applied ORs and ORs + N fertilizers. Nevertheless, Nayak et al. (2009) observed greater microbial biomass C and N, and active SOC with ORs + N fertilizers than ORs.
Greater yield responses with sole N fertilizer than OR probably also led to greater belowground C inputs in the sole N fertilizer (Fig. 2), but N fertilizers applied alone did not increase SOC (Fig. 12a). Similarly, Gentile et al. (2010) and Khan et al. (2007) observed greater SOC contents in treatments where OR was added than when N fertilizer was added. In contrast, Goyal et al. (1992) found that the addition of N and P fertilizers resulted in increased SOC contents. In their study, they also observed greater SOC contents following the combined application of OR and N fertilizers and this was mainly attributed to increased root growth associated with greater organic matter inputs. Studies that compared above- versus belowground C input to soil generally find a greater stabilization of root-derived C than residue-derived C (Denef and Six 2006; Gale and Cambardella 2000; Gale et al. 2000; Six et al. 2002). Nevertheless, our results further emphasize the need of adding C inputs in order to build up and/or maintain SOC, especially in sandy soils.
Given that the studies included in the meta-analysis were carried out across most agro-ecological zones in SSA and across different soil textures using a wide range of OR qualities, we can confidently conclude that yield responses are largely dependent on addition of nutrients, soil texture and MAP. While OR quality clearly influences crop yield responses, the use of polyphenol content to separate classes I and II seems to be of less importance under field conditions. Organic resource N content and total quantities of OR-N added, just like total fertilizer-N added, are more influential on crop yields than polyphenol contents. Yield responses were greater following the combined application of ORs with N fertilizers compared to the addition of either resource alone. However, the dominance of negative interactive effects and lack of differences in agronomic N use efficiencies with ORs applied alone but lower than N fertilizers applied alone imply that the extra increase in grain yield is mostly related to the extra N added and not to an increase in efficiency of utilization by applying the resources together. However, given that total N added in the combined treatment was ≥100 kg N ha-1 in 70% of the observations, possible interactive effects could have been masked with reduced resource utilization efficiency. Utilization efficiency of combining ORs with N fertilizers were greater when lower quantities of OR-N and fertilizer-N were added, suggesting that this may be the most appropriate strategy for managing the resources, particularly in sandy soils. Absolute yield responses were greater in finer textured soils and high MAP areas, but the proportional increases to nutrient additions were greater in low MAP areas and coarse textured soils. Therefore, positive interactive effects were observed when low quality OR was incorporated in sandy soils. Furthermore, there were no residual effects of sole ORs in sandy soils on yield, showing the need to continuously add N fertilizers in these soils. In addition, the application of ORs increased SOC, and this was more so in sandy soils, and thus offers, in combination with N fertilizer additions, the potential for the improvement of soil quality, crop productivity and its sustainability in SSA.
This study was funded by the National Science Foundation, Ecosystem Cluster (DEB: 0344971). We are also grateful to F. Mtambangwe, P. Mapfumo and N. Nhamo for sharing their data; K. J. van Groenigen and M.A. de Graaf for their advice on statistical procedures.
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