Decomposition and nutrient release pattern of wheat (Triticum aestivum) residues under different treatments in desert field conditions of Sudan

Recycling of crop residues is essential to sustain soil fertility and crop production. Therefore, it is of crucial importance to study the decomposition of crop residues particularly in the arid tropics. The decomposition and nutrient release pattern from crop residues incorporated in the soil have rarely been investigated under semi-arid climatic conditions in Sudan. Decomposition and nutrient release pattern from wheat residue were investigated in a 12 week litter bag experiment under field condition. Litter bags contain 50 g wheat residue were buried in plots cultivated with guar and treated with following treatments: crop residue (CR), recommended fertilizer + crop residue (RF + CR), sewage sludge (SS) and non treated control (C). In each plot of each block, 12 litter bags were buried. Four bags from each treatment were retrieved at 2, 4, 6, 8, 10 and 12 weeks of decomposition. The decomposed residue was analysed for remaining dry matter (DM), nitrogen (N), phosphorus (P), potassium (K) calcium (Ca), and magnesium (Mg) contents. Mass loss and nutrients released from wheat residue followed the exponential model, Wt = W0 × e−kt from which the specific decay rate constants (k) and t0.5 and t0.95 were calculated. The result show that the mineral fertilization combined with crop residues (RF + CR) resulted in the maximum decomposition and nutrient release compared to the other treatments. The half-life (t0.5) values of the treatments were in the following order: RF + CR (6.70 weeks) < SS (8.0 weeks) < CR (9.60 weeks) < C (11.91 weeks). The percentage remaining DM in the litter bags followed the order C (73.25 %) > CR (66.45 %) > SS (59.85 %) > RF + CR (53.85 %). A 50 % loss of residue N was found after 8.88 weeks in the C treatment and 5.04 weeks in RF + CR treatment. Generally, nutrient loss from all treatments in the order of Mg > K = N > Ca > P. Application of mineral fertilizer or sewage sludge enhances the decomposition of low-quality residue (wheat). Therefore, it is essential to apply fertilizer in degraded soil before incorporation of crop residue with high carbon to nitrogen ratio.

they contain. Crop residue decomposition is affected by the physical and chemical characteristic of both crop residue and the soil where the crop residue is incorporated. Kriaučiūnien_ e et al. (2012) reported that C:N ratio and lignin concentration are most important factors affecting the decomposition of incorporated crop residue. Swift et al. (1979) suggest that availability of soil nutrients is one of the factors affecting the rate of litter decomposition. However, reported results from the effects of increased soil N and P on the rate of litter decomposition and nutrient dynamics are inconsistent. The effect of increased soil N and P on the rate of litter decomposition varied from no effect (Hobbie and Vitousek 2000) positive effects on decomposition (Vestgarden 2001) and negative effects (Magill and Aber 1998) Several studies have addressed litter decomposition under field conditions of humid temperate climate (Joergensen et al. 2009;Robinson et al. 1994), boreal, and wet tropical conditions (Tam et al. 1990;Saini 1989). Plurality of these studies have been carried out in forest soils under semi natural conditions (Xianiu and Hirata 2005;Alhamed et al. 2004). Fertilizer management and cultivation of different crops may have strong effects on the decomposition of plant residues, due to the difference in nutrient inputs by the fertilizers and the difference in nutrient demand by the crops.
Wheat residue used in this experiment had a low N content, and thus, the activity and the growth of microorganism were limited by the availability of N. An increase in the size of the soil microbial biomass is considered essential for soil fertility improvement (Singh et al. 2006). Kwabiah et al. (1999) found that application of fertilizer urea or triple super phosphate enhanced the decomposition of litter from Croton megalocarpus and Sesbania sesban in the early stages of decomposition compared to control. Liu et al. (2006) noted that adding N alone or in combination with P stimulated the decomposition rate of Allium bidentatum and Stipa krylovii, which can be explained by the consequent increases in the availability of N and P, which are considered the main determinants of mass and nutrient loss during decomposition (Alexander 1977). As inputs of inorganic N or P increased soil N and P contents, microorganism activities may become limited more by carbon availability than by the available N and P. Under such conditions, C may be released from the decomposing litter and used as an energy source for catabolic and anabolic activities, resulting in further increases in rates of litter breakdown (Muhammad et al. 2011).
As for biological effects on soil, application of sewage sludge increased soil microbial biomass C and soil respiration (an index of general metabolic activity of soil microorganisms) when compared to a control (Bhattacharyya et al. 2003). Soil enzymes are extensively used as biological assay in soil. Perucci (1990) found that after application of 75 Mg ha -1 of municipal sludge compost increased the enzyme activities of phosphodiesterase, alkaline phosphomonoesterase, arylsulphatase, deaminase, urease and protease.
Synchronizing soil nutrients availability with plant requirements can improve the soil-plant system, N use efficiency and reduce nutrients losses through leaching below the crop rooting depth and/or from gaseous emissions (Vanlauwe et al. 2001).
Litter bag method developed by Bocock and Gilbert (1957) is used to study decomposition processes of plant residues under field conditions. Although this method creates a microenvironment, it is still considered to be a useful approach, due to the relatively simple recovery of litter transferred to the field and the possibility of excluding specific fauna groups from the decomposition process (Joergensen et al. 2009;Knacker et al. 2003). Litter bags have been less intensively used in studying the decomposition of green manure and harvest residues in arable areas (Knacker et al. 2003;Berg 2000;Christensen 1985).
Decomposition and nutrient release from the crop residues with organic and inorganic fertilizers have rarely been studied in Sudan. Therefore, the objective of this research was to determine rate of decomposition and nutrient (N, P, K, Ca, Mg) release patterns resulting from decomposition of wheat residues in a field cultivated with guar Cyamopsis tetragonoloba and treated with organic and inorganic fertilizers.

Site, soil, and climate
Field experiment to determine the decomposition of wheat residues with litter bags was carried out from June to August 2010 in the main field experiment of Omdurman Islamic University, Sudan (15°19.9N, 32°39 0 E, and with an elevation of 381 m above the sea level). The study was conducted for five seasons (wheat-guar-wheat -gaurwheat) to study the contribution of organic residues in sustaining yields of wheat (Triticum aestivum var. Imam.) and guar (Cyamopsis tetragonoloba local var.) in a crop rotation system (Rezig et al. 2013). Treatments included recommended inorganic fertilizer 92 kg P 2 O 5 ha -1 (204.4 kg triple super phosphate ha -1 ) and 125 kg N ha -1 (271.7 kg urea ha -1 ) with crop residues (RF ? CR), crop residues (CR), 10 t ha -1 sewage sludge (SS) and control (C). The whole plant residue was used in this study after removing the roots. The soil properties and chemical composition of wheat residue were given in Tables 1 and 2. Table 1 Some chemical properties of the experimental site  Litter bag experiment Decomposition (mass loss) of the wheat, residues and nutrient release were monitored in the field using the litter bags method (Anderson and Ingram 1989). Nylon bags of 20 9 10 cm dimensions with a mesh size of 2 mm were used for this study. The straw of the wheat was cut into length of about 20 cm and 50 g of oven-dried crop residues were placed into each litter bag. One side of the bag was left open for faunal activity. Ninety-six litter bags were used for this study. The litter bags from wheat residue were distributed in the control (C), crop residue at the rate of 2.5 t ha -1 (CR), recommended fertilizer ? crop residue at the rate of 6.63 t ha -1 (RF ? CR) and sewage sludge (SS) treatment. In each plot of each block, 6 l bags were buried. The litter bags were numbered and buried 5 cm below the ridge top to simulate residue incorporation into the plough layer. The growing guar plants were left standing for the duration of the experiment for 3 months and harvested manually in September 2010. The experiment was terminated 7 days after crop harvest. At each sampling 2, 4, 6, 8, 10 and 12 weeks after application of the residues 4 l bags from each of the four treatments (16 l bags per sampling) were retrieved and soil attached to the plant part was carefully removed. Each bag was placed inside a paper envelope and transferred to the laboratory for analysis. The content was emptied and extraneous materials, such as soil, visible animals and fine roots were removed. The decomposing crop residues were oven-dried at 65-70°C for 48 h or constant weight and weighed to determine litter mass loss rates (Anderson and Ingram 1989). The remaining non-decomposed materials were ground to pass 1 mm sieve for chemical analysis. The ground samples were analysed for Nitrogen (Bremner and Mulvaney 1982), P, K, Ca and Mg (Chapman and Pratt 1961).

Statistical analysis
The percentage of dry mass remaining (DM) in each bag was calculated using the following equation: where W t = Weight remained after each sampling week (g) and, W 0 = Initial weight (g) which is potentially decomposable The data for dry mass remaining from each treatment were fitted to a negative exponential model (Olson 1963): Exponential decomposition models have been extensively used to describe the decomposition of litter in litter bags (Patricio et al. 2012): where W 0 = initial residue weight at time zero, W t = litter remaining after a given time (t), t = time interval of sampling expressed in weeksm, k = rate constant (decomposition rate constant per week) and, e = base of natural logarithms Nutrients content (N, C, P, K, Ca 2? and Mg 2? ) of decomposing crop residue was determined by the following equation: where W t is the remaining mass at time t (in weeks), W 0 is the initial weight of the litter, C t is the concentration of element in decomposing litter at the time of sampling and C 0 the initial concentration of element. The regressions of ln (W t /W 0 ) over time were performed separately for each set of litter bags in each plot to provide independent estimates of k and R 2 for each treatment. Further, the time required for 50 % (t 0.5 ) and 95 % (t 0.95 ) decay was calculated as t 0.5 = 0.693/k and t 0.95 = 3/k, respectively. Statistical analysis software (SAS 1985) was used to test variations between treatments and least significant difference (LSD) was used to determine differences between treatment means.

Mass loss
Prior treatment of the field plots with organic and inorganic fertilizers significantly affected the amount of wheat residue lost during incorporation (Fig. 1). After 12 weeks of incorporation, wheat residue had lost 26.8, 33.6, 40.2 and 46.2 % of its initial mass in the C, CR, SS and RFCR plot, respectively. The data on decomposition rate constants (k) values, R 2 and the days taken for 50 and 95 % of residues to decompose (t 0.5 and t 0.95 ) were presented in Table 3. The half-life (t 0.5 ) values (i.e. the time for the wheat residue to lose half their initial mass) of the treatments were in the following order: RF ? CR (6.70 weeks) \ SS (8.0 weeks) \ CR (9.60 weeks) \ C (11.91 weeks). Generally, carbon decomposition followed the same trend as dry matter disappearance.  Fig. 3. Generally, release of P during the study period was slow compared to nitrogen in all treatments (Table 3). After 2 weeks, 84.94, 86.85, 95.88 and 96.75 % of the initial P remained in the SS, RF ? CR, CR and C treatments, respectively. Fifty per cent of P remained of the treatments were in the following order: RF ? CR (8.41 weeks) \ SS (9.75 weeks) \ CR (10.91 weeks) \ C (16.83 weeks). At the end of the study period, 37.45, 41.62, 48.32 and 62.1 % of the initial P contents were remaining for RF ? CR, SS, CR and C treatments, respectively. The highest decomposition rate constant (0.083 week -1 ) in RF ? CR treatment indicates of fast decomposition relative to other treatments. Generally, the release of K in all treatments was higher than that of N and P (  (Fig. 4). Fifty per cent of K remained from wheat residue were in the following order: RF ? CR (5.51 weeks) \ SS (6.30 weeks) \ CR (9.00 weeks) \ C (13.28 weeks). At the end of the study period, 21.02, 23.97, 34.75 and 50.08 % of the initial K content were remaining in RF ? CR, SS, CR and C treatment, respectively.
Treatments significantly increased the decomposition rate of both Ca and Mg from wheat residue. The release of Ca was slow in all the treatments compared to other nutrients (Table 3). RF ? CR treatment decomposed 50 % of its initial Ca and Mg content at the 7.37, and 4.41 weeks, respectively. At week 12, 60.86, 48.86, 38.73 and 33.26 % of the initial Ca contents were remaining in C, CR, SS and RF ? CR treatments, respectively (Fig. 5). After 2 weeks, the Mg remaining in RF ? CR treatment (89.11 %) was  significantly (P B 0.0002) lower than that in all treatments. The half-life (t 0.5 ) values of the treatments were in the following order: RF ? CR (4.14 weeks) \ SS (4.9 weeks) \ CR (8.12 weeks) \ C (9.28 weeks). The per cent Mg remaining (Fig. 6)

Discussion
Incorporation of crop residue with or without fertilizer and sewage sludge significantly changed soil physicochemical properties (Rezig et al. 2013). The decomposition rate of wheat residue increased with the increase in N and P content in the soil. The initial N and P concentration in the plots was in the following order:  Table 1). The increase in the decomposition of wheat residue with the increase in soil nutrients is consistent with previous study report that increase in nutrient supply (N and P) increased litter decomposition (Thirukkumaran and Paskinson 2002). The decomposition of plant litter is limited by the concentration of inorganic N and P (Alexander 1977). The increase in decomposition of wheat residue may be attributed to the increase in microbial biomass and efficiency (Thirukkumaran and Paskinson 2002;Qualls and Richardson 2000); increase in degradation of cellulose ; change in soil microorganisms community; increase in microbial activity (Gulis and Suberkropp 2003) and increase in extracellular enzymes such as Urease, acid phosphatase and glycosidase in the soil and in the litter (Saiya-Cork et al. 2002). However, the finding of the current study is in contrast with result reported by Chen et al. (2013). They reported that addition N and P decreased litter decomposition. They attributed the negative effect of N addition on litter decomposition to related high N-saturation in the soil and the negative effect of P addition to the suppression of microbial P mining. Knorr et al. (2005) concluded that N addition may stimulate litter decomposition in N-limited forest, but inhibit litter decomposition in N-rich forest. Pernin et al. (2006) reported that application of sewage sludge increased mesofauna population in treated plots. Similarly, Bhattacharyya et al. (2003) found that application of sewage sludge increased soil microbial biomass C and soil respiration when compared to a control. Application of 75 Mg ha -1 of municipal sludge compost increased the activity of enzyme such as phosphodiesterase, alkaline phosphomonoesterase, arylsulphatase, deaminase, urease and protease (Perucci 1990).
The lower nitrogen remaining from incorporated wheat residue in RF ? CR and SS treatment compared to Cr and C treatment might be attributed to their higher N content (Table 1). Increase in soil N improved the activities of microorganisms, which enhanced the transformation process in soil including the decomposition of plant residues and accumulation of N in the soil (Bhattacharyya et al. 2008;Anwar et al. 2005). Addition of N to the soil accelerates the decomposition and release of nutrients from material with high C:N ratio . The release of P from wheat residue increased with the increase in the soil P in the soil. This result was similar to the pattern of N released.
Fertilization with sewage sludge or crop residue with fertilizer significantly (P \ 0.0001) enhances K decomposition. The fast release of K in all treatment relative to other nutrient may be attributed to its existence in the cell fluid as non structural components (Christensen 1985). Similar results were reported by Mubarak et al. (2002); Attiwell (1968);Christensen (1985).
Similar trend was observed for the release of Ca and Mg. The release of Ca and Mg is related to mass loss trend. Therefore, increase in mass loss with the increase in nutrient content in the soil resulted in increase in Ca and Mg decomposition rate from wheat residue. Hodges (2010), Demarty et al. (1984) suggest that Ca is structural component of plant cell.

Conclusions
Soil nutrient contents have strong effects on the decomposition of low-quality residue (wheat residue). Mineral fertilization combined with crop residues significantly enhanced decomposition and nutrient release. Application of N and P is important management practice to enhance the decomposition of high C:N ratio crop residue in the field. the field and laboratory work and drafted the manuscript. Elhadi, E.A participates in several field works, statistical analysis and participates in correction of the manuscript. Mubarak, A. R He is the main supervisor and drafts the proposal and participates in the correction of the manuscript. All authors read and confirmed the final manuscript. Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.