Dynamics of soil nitrogen availability indices in a sandy clay loam soil amended with animal manures

Abstract

Purpose

Transformation of nitrogen (N) from different manure sources to available forms to promote food security in context of rising climate change is vital. Effect of manure (types, nutrients, high rates and application time) on soil N transformation requires further investigation. This study evaluated effects of three animal manures on soil N mineralization of sandy clay loam.

Methods

A 22-week field study in a Randomized Complete Block Design (three replicates) was conducted using dried Cattle, Goat and Poultry Manures (CGPM), applied at 5, 10, 20, 40, 60, 80, 120 and 150 t ha⁻¹ once at onset of the study. Also, N₁₅P₁₅K₁₅ (0.4 t ha⁻¹) was incorporated in three splits of 2, 8 and 14 weeks after incorporation (WAI) of manures as reference. Soil NH₄ ⁺–N and NO₃ ⁻–N were determined bi-weekly.

Results

The results showed N release peaked at 10 WAI with highest NH₄ ⁺–N (830, 400, 253 mg kg⁻¹) and NO₃ ⁻–N (316, 398, 250 mg kg⁻¹) at 150, 60 and 60 t ha⁻¹ for CGPM, respectively. Initial rapid N release phase occurred at 0–4 WAI, NH₄ ⁺–N and NO₃ ⁻–N increased averagely by 182, 183, 139% and 131, 175, 144% for CGPM, respectively. Declines occurred at 8, 14 and 22 WAI but reduction observed at 22 WAI tripled 8 and 14 WAI.

Conclusions

Application of NPK and all the manures provided highest soil NH₄ ⁺–N and NO₃ ⁻–N at 12 WAI. High rates of CGPM were not injurious to these soil properties; hence this has implication for N to maximize plants uptake as well as decrease losses to environment.

Introduction

Agricultural growth as a result of population increase had contributed to intensification of land cultivation; this causes severe soil fertility depletion. However, losses of nutrients in the soil have been replenished through the use of animal manures (Ramsasa 2010). Hence, the potential of animal manure use in soil replenishment has received the global attention. Currently, to many soil fertility difficulties, animal manures offer an affordable and readily available solution (Spore 2006) and are important source of Ca, Mg, S and micronutrients (Gomez-Brandon et al. 2013); they contain low and highly variable amounts of N, P and K. Aside from being source of plant nutrients, the physical properties of soil are also improved (Akanni and Ojeniyi 2008). However, the use of animal manures can pose a major problem of excess nitrate in soil (Azeez and Van Averbeke 2010b; Navarro Pedreno et al. 1996). The difficulty in the prediction of manure nutrients to plants is due to turnover processes and losses in manures (Azeez and Van Averbeke 2010a; Sorenson 2001). Therefore, incorporation of animal manures to supply nitrogen in proper dose at the proper time without posing any injury to soil and plants is very important in soil replenishment to support crop productivity. Hence, in order to predict the net mineralization of nitrogen in animal manures, transformation of nitrogen from organic into inorganic form and immobilization processes need to be understood. These processes are mediated through microbial activities and naturally, they are biochemical (Vel and Swamam 2013; Bartholomow 1965). Large variability in manure qualities (Chadwick et al. 2000) or types had posed difficulties for researchers in concluding on a specific rate of manure to support development of different crops. The consequence of these led to differences in the recommendations of 60 t ha⁻¹ of cattle kraal manure for Corchorus olitorius L. (Masarirambi et al. 2012b), goat manure rate of 20 t ha⁻¹ for sweet maize (Uwah and Eyo 2014); and poultry manure at the levels of 60 t ha⁻¹ (Masarirambi et al. 2012a) and 30 t ha⁻¹ (Enujeke et al. 2013) for lettuce and maize yield, respectively.

Animal manures at proper application time, adequate rates and proper nutrient contents when expanding the uses of organic matter in place of inorganic fertilizers for nitrogen mineralization to meet crop N requirement without adverse problem on soil, crop and the environment need to be investigated. This necessitated the study to (1) examine the release patterns of NH₄ ⁺–N and NO₃ ⁻–N from different rates of three animal manures and recommended inorganic fertilizer rate to soil, (2) evaluate the potential rates of mineralization of the three animal manures on soil and (3) determine the influence of high application rates of the three animal manures on soil.

Materials and methods

Location of the experiment

This research was conducted at the Teaching and Research Farm of the Federal University of Agriculture, Abeokuta, Ogun State, Nigeria, located between latitude 7°12′ N and longitude 3°20′ E (FUNAAB 2013). Irregularities of rainfall occurred during the period of the study (Table 1).

Table 1 Agrometeorological data for the period of the experiment

Experimental design

The experiment was a randomized complete block design (RCBD) with three replicates. Cattle, goat and poultry manures were applied at the rates of 5, 10, 20, 40, 60, 80, 120 and 150 t ha⁻¹ separately; these were used to test the pattern of manure mineralization irrespective of the rates, to test the manure-induced soil factors that affect plant growth and also evaluate the residual effect of manure rates on soil properties. NPK 15–15–15 fertilizer was used as check for each manure treatment and applied at the rate of 400 kg ha⁻¹ (Schippers 2000). Starting from 2 weeks after incorporation of manures but a day after application of NPK fertilizer, soil samples were collected at every 2-week intervals from the net portion of each bed at a depth of 0–15 cm with the exception of 6 and 16 WAI due to rainfall failure. The samples were air dried, prepared for laboratory analysis and analyzed for NH₄ ⁺–N and NO₃ ⁻–N.

Soil and manures analyses

Initial soil sample and those collected bi-weekly were analyzed using the following methods. Soil EC and pH were extracted with 1:2 soil:water ratio and measured potentiometrically using HANNA 215 electrical conductivity meter (Richards 1954) and glass pH meter electrodes (HANNA pHep), respectively, (Van Reeuwijk 1993). The NH₄ ⁺–N and NO₃ ⁻–N were extracted in 2 Molar K₂SO₄ and determined colorimetrically using the method of Okalebo et al. (1993). Available P was extracted by Bray P-1 extraction (Bray and Kurtz 1965), obtained colorimetrically (Murphy and Riley 1962). Exchangeable bases were extracted with 1 N Ammonium-acetate solution in 1:10 soil solution ratio, K⁺ and Na⁺ were analyzed with flame photometer, while Ca²⁺ and Mg²⁺ with Atomic Absorption Spectrophotometer (AAS) (Anderson and Ingram 1993). Organic carbon (OC) content was determined by Walkley–Black method (Nelson and Sommers 1990) and particle size by hydrometer method (Bouyoucos 1965).

The cattle, goat and poultry manures used were digested with nitric per chloric acid (2:1) (Watanabe et al. 2013; Silva and QueirÓz 2002). The digests were analyzed for macronutrients and micronutrients using standard procedures (Kaira and Maynard 1991; Cater 1993). Distilled water was used for extraction to determine the electrical conductivity and pH of the manures.

Statistical analysis

Data collected were subjected to Analysis of Variance (ANOVA) using SAS (1999). Duncan Multiple Range Test at 5% level of probability was used to determine differences in the treatment rates means and also to show the significance effects on parameters measured in relation to the control.

Results

Agrometeorological data for the experimental period

Agrometeorological average data for rainfall, relative humidity, mean temperature, soil temperature, mean sunshine hour and evaporation at 2-week intervals during the experiment were presented in Table 1.

Soil characteristics

The result revealed that the soil used for the research had EC value of 0.69 dS m⁻¹, pH of 7.6 and total N value of 0.8 g kg⁻¹. The NH₄ ⁺–N, NO₃ ⁻–N, OC and available P values were 0.013, 0.014, 14.2 g kg⁻¹ and 7.5 mg kg⁻¹, respectively. Exchangeable bases; K⁺, Na⁺, Ca²⁺ and Mg²⁺ had the values of (0.42, 0.82, 10.96 and 1.34) cmol⁺ kg⁻¹, respectively. The carbon to nitrogen ratio (C: N) was 11.83. The micro-nutrients; Mn²⁺, Fe²⁺, Cu²⁺ and Zn²⁺ had the values of (55.1, 9.85, 1.2 and 5.1) mg kg⁻¹, respectively. The percentages of sand, silt and clay were 770, 68 and 162 g kg⁻¹, respectively.

Manures characteristics

The EC values of the cattle, goat and poultry manures were 12.4, 8.8 and 16.4 dS m⁻¹, respectively, while the pH values of the cattle, goat and poultry manures were 7.9, 7.7 and 7.8, respectively. The equivalent amount of total N, NH₄ ⁺–N, NO₃ ⁻–N, P, K, Na, Ca, Mg and OC in cattle manure added to the soil between the rates of 5–150 t ha⁻¹ were 225.5–6765, 2.1–63, 2.2–66, 16–480, 37–1110, 46.5–1395, 148–4440, 47.5–1425 and 894–26,820 kg, respectively. However, addition of goat manure between the rates of 5 and 150 t ha⁻¹ is equivalent to adding total N, NH₄ ⁺–N, NO₃ ⁻–N, P, K, Na, Ca, Mg and OC of 241–7230, 1.6–48, 1.8–54, 3.5–105, 18.5–555, 23–690, 90.5–2715, 50–1500 and 1115–33,450 kg, respectively, to the soil. Hence, the equivalent amount of total N, NH₄ ⁺–N, NO₃ ⁻–N, P, K, Na, Ca, Mg and OC in the poultry manure applied to the soil at the rates ranged from 5 to 150 t ha⁻¹ estimated to the amount of 139–4170, 7.4–220.5, 7.5–223.5, 81–2430, 19.5–585, 24.5–735, 88.5–2655, 26.5–795 and 296.5–8895.0 kg, respectively (Tables 2, 3, 4).

Table 2 Equivalent amount of nutrients present in cattle manure rates added to the soil

Table 3 Equivalent amount of nutrients present in goat manure rates added to the soil

Table 4 Equivalent amount of nutrients present in poultry manure rates added to the soil

Effect of cattle, goat and poultry manures on soil NH₄ ⁺–N and NO₃ ⁻–N (mg kg⁻¹)

Effect of cattle manure on soil NH₄ ⁺–N (mg kg⁻¹)

Table 5 reveals that at 2 WAI, soil applied with cattle manure rate of 60 t ha⁻¹ gave highest value of 62.06 mg kg⁻¹ in NH₄ ⁺–N and lowest of 27.69 mg kg⁻¹ at 80 t ha⁻¹ by 171 and 21%, respectively, above the control, whereas percentage decreases of 13, 9 and 0.04% were recorded in soil with cattle manure rates of 120, 150 t ha⁻¹ and NPK, respectively. But there were no significant differences in control and the amendment rates at 4 WAI. While at 8 WAI, soil with cattle manure rate of 120 t ha⁻¹ had NH₄ ⁺–N of 51.66 mg kg⁻¹ with the highest increase of 98%, soil applied with 20 t ha⁻¹ of cattle manure had 36.50 mg kg⁻¹ of NH₄ ⁺–N with the lowest increase of 40% in relation to control. The soil with cattle manure rate of 150 t ha⁻¹gave highest NH₄ ⁺–N value of 830.4 mg kg⁻¹ by 636% but 5 t ha⁻¹ resulted in the least value of 221.4 mg kg⁻¹ by 96% relative to control at 10 WAI. However, the amendment rates were not significantly different from control soil at 12 WAI. At 14 WAI, highest increase of 61% and lowest increase of 14% above the control were obtained from soil applied with cattle manure rates of 150 and 40 t ha⁻¹ with NH₄ ⁺–N values of 29.32 and 20.88 mg kg⁻¹, respectively. While 5, 10, 20 t ha⁻¹ and NPK decreased by 38, 29, 31 and 72%, respectively relative to control. At 18 WAI, soil with cattle manure rate of 150 t ha⁻¹ with NH₄ ⁺–N value of 86.55 mg kg⁻¹ recorded the highest increase of 290%, but 10 t ha⁻¹ had 23.33 mg kg⁻¹ and recorded the least increase of 5% over the control, while 5 t ha⁻¹ with 21.11 mg kg⁻¹ of NH₄ ⁺–N decreased by 5%. At 20 WAI, soil with cattle manure rate of 150 t ha⁻¹ gave 49.53 mg kg⁻¹ of NH₄ ⁺–N and had the highest increase of 111% while the rate of 5 t ha⁻¹ had 25.03 mg kg⁻¹ of NH₄ ⁺–N and recorded the lowest increase of 7% relative to control. But the rate of 60 t ha⁻¹ with NH₄ ⁺–N value of 5.26 mg kg⁻¹ recorded the highest increase of 149%, while 20 t ha⁻¹ and 120 t ha⁻¹ with NH₄ ⁺–N value of 2.99 mg kg⁻¹ each had the least increase of 42% compared to control at 22 WAI. However, the level of soil NH₄ ⁺–N decreased in relation to the corresponding rates at 20 WAI.

Table 5 Influence of cattle manure rates and NPK fertilizer (t ha⁻¹) on mean soil NH₄ ⁺–N (mg kg⁻¹) at 2 weeks intervals

Effect of goat manure on soil NH₄ ⁺–N

Table 6 showed that aside the soil applied with goat manure rate of 150 t ha⁻¹ that was significantly higher by 114% with NH₄ ⁺–N of 48.27 mg kg⁻¹ at 2 WAI, soil with other amendment rates were not statistically different from control. The soil with goat amendment rates followed similar trend as cattle treatment at 4 WAI. The soil NH₄ ⁺–N increased in comparison with the corresponding rates at 2 WAI. At 8 WAI, soil with goat manure rate of 150 t ha⁻¹ gave NH₄ ⁺–N value of 44.17 mg kg⁻¹ and recorded the highest increase of 53% while 20 t ha⁻¹ had NH₄ ⁺–N value of 34.60 mg kg⁻¹ and with the least increase of 20% above the control. But reductions of 18 and 28% were obtained from 5 to 10 t ha⁻¹, respectively. The level of NH₄ ⁺–N increased relative to corresponding rates at 4 WAI. The soil applied with goat manure rate of 60 t ha⁻¹ resulted in NH₄ ⁺–N value of 400.3 mg kg⁻¹ with highest increase of 300% while 10 t ha⁻¹ had 225 mg kg⁻¹ and recorded the least increase of 126% relative to control at 10 WAI. But 12 WAI followed the same trend as soil applied with cattle amendment. Furthermore, soil NH₄ ⁺–N decreased in comparison with the corresponding rates at 10 WAI. Highest increase of 24% was recorded from soil with goat manure rate of 150 t ha⁻¹ which had NH₄ ⁺–N value of 22.88 mg kg⁻¹ at 14 WAI. The soil with rate of 60 t ha⁻¹ resulted in 18.67 mg kg⁻¹ and gave the lowest increase of 1% more than the control. However, 5, 10 t ha⁻¹ and NPK reduced by 29, 13 and 70%, respectively. The level of soil NH₄ ⁺–N decreased in relation to the corresponding rates at 12 WAI. However, soil applied with goat manure rate of 150 t ha⁻¹ had highest NH₄ ⁺–N value of 88.77 mg kg⁻¹ by 300% while 5 t ha⁻¹ resulted in 26.63 mg kg⁻¹ and recorded the least increase of 20% at 18 WAI, compared with control. At 20 WAI, soil with goat manure rate of 120 t ha⁻¹ recorded NH₄ ⁺–N value of 47.76 mg kg⁻¹ and had the highest increase of 105%. But the soil with goat manure rate of 5 and 10 t ha⁻¹ with the NH₄ ⁺–N value of 24.45 mg kg⁻¹ each recorded the lowest increase of 5%. However, there was a decrease in soil NH₄ ⁺–N level in relation to the corresponding rates at 18 WAI. Consequently, the soil with goat manure rate of 120 t ha⁻¹ which gave NH₄ ⁺–N value of 4.99 mg kg⁻¹ recorded the highest increase of 141% at 22 WAI while 5 t ha⁻¹ resulted in 3.04 mg kg⁻¹ with the least increase of 47% relative to control. There were decreases in soil NH₄ ⁺–N level compared with the corresponding rates at 20 WAI.

Table 6 Influence of goat manure rates and NPK fertilizer (t ha⁻¹) on mean soil NH₄ ⁺–N (mg kg⁻¹) at 2 weeks intervals

Effect of poultry manure on soil NH₄ ⁺–N

At 2 WAI, soil applied with poultry manure rate of 60 t ha⁻¹ gave NH₄ ⁺–N value of 50.86 mg kg⁻¹ and recorded the highest increase of 153%. The rate of 10 t ha⁻¹ had soil NH₄ ⁺–N value of 20.18 mg kg⁻¹ and had the least increment of 0.5% more than the control. But the values at 4 WAI followed similar trend with soil on which cattle and goat manures were incorporated. The soil NH₄ ⁺–N increased compared to the corresponding rates at 2 WAI. Meanwhile, soil applied with poultry manure rate of 150 t ha⁻¹ recorded highest soil NH₄ ⁺–N value of 47.85 mg kg⁻¹ by 90% at 8 WAI. However, soil on which NPK was added with NH₄ ⁺–N value of 37.21 mg kg⁻¹ recorded the least increase of 47% over the control. The level of soil NH₄ ⁺–N decreased in relation to the corresponding rates at 4 WAI. More so, soil with poultry manure rate of 60 t ha⁻¹ resulted in 253.4 mg kg⁻¹ of NH₄ ⁺–N and had the highest increase of 138%, while the rate of 5 t ha⁻¹ with soil NH₄ ⁺–N value of 110.7 mg kg⁻¹ recorded the least increase of 4% relative to control at 10 WAI. The soil applied with poultry amendment rates were not differerent significantly from control at 12 WAI but soil NH₄ ⁺–N level decreased relative to corresponding rates at 10 WAI. At 14 WAI, soil with poultry manure rate of 150 t ha⁻¹ had NH₄ ⁺–N value of 28.06 mg kg⁻¹ and recorded the highest increase of 49% but 40 t ha⁻¹ resulted in 19.86 mg kg⁻¹ of NH₄ ⁺–N with lowest increase of 6% more than the control. However, at 14 WAI the level of soil NH₄ ⁺–N decreased in relation to the corresponding rates at 12 WAI. At 18 WAI, soil with poultry manure rate of 150 t ha⁻¹ gave highest NH₄ ⁺–N value of 73.23 mg kg⁻¹ by 230% increase while 5 t ha⁻¹ had least NH₄ ⁺–N value of 23.27 mg kg⁻¹ by 5% increase compared with control, whereas, 80 t ha⁻¹ resulted in soil NH₄ ⁺–N value of 52.44 mg kg⁻¹ and had the highest increase of 127% more than the control at 20 WAI while 20 t ha⁻¹ with soil NH₄ ⁺–N value of 27.41 mg kg⁻¹ recorded the least increase of 19%. At 22 WAI, soil with poultry manure at the rate of 150 t ha⁻¹ with NH₄ ⁺–N value of 4.45 mg kg⁻¹ recorded the highest increase of 119% while 5 t ha⁻¹ gave soil NH₄ ⁺–N value of 2.08 mg kg⁻¹ and recorded the least increase of 3% above the control. In addition, soil NH₄ ⁺–N values decreased in comparison with the corresponding rates at 20 WAI, Table 7.

Table 7 Influence of poultry manure rates and NPK fertilizer (t ha⁻¹) on mean soil NH₄ ⁺–N (mg kg⁻¹) at 2 weeks intervals

Effect of cattle manure on soil NO₃ ⁻–N

The soil applied with cattle manure rate of 60 t ha⁻¹ resulted in NO₃ ⁻–N value of 54.09 mg kg⁻¹ at 2 WAI by 119% while the soil with NPK had the NO₃ ⁻–N value of 25.98 mg kg⁻¹ and recorded the least increase of 5% more than the control. The 4 WAI was in line with soil NH₄ ⁺–N but the soil NO₃ ⁻–N values increased compared with the corresponding rates at 2 WAI. However, at 8 WAI, soil with cattle manure at the rate of 120 t ha⁻¹ gave NO₃ ⁻–N value of 50.42 mg kg⁻¹ with highest increase of 117% but 10 t ha⁻¹ with soil NO₃ ⁻–N value of 25.56 mg kg⁻¹ recorded the least increase of 10% in relation to control. However, the soil NO₃ ⁻–N values reduced in comparison with the corresponding rates at 4 WAI, Table 8. Moreover, soil with cattle manure rate of 150 t ha⁻¹ with soil NO₃ ⁻–N of 316.68 mg kg⁻¹ resulted in highest increase of 201% at 10 WAI while 5 t ha⁻¹ with 133.78 mg kg⁻¹ had the least increase of 27% in relation to control. At 12 WAI, 80 t ha⁻¹ with soil NO₃ ⁻–N value of 139.60 mg kg⁻¹ gave highest increase of 52% but 5 t ha⁻¹ had soil NO₃ ⁻–N value of 97.70 mg kg⁻¹ and had lowest increase of 7% relative to control. However, soil NO₃ ⁻–N decreased relative to the corresponding rates at 10 WAI. The soil with cattle manure rate of 120 t ha⁻¹ gave highest NO₃ ⁻–N value of 26.42 mg kg⁻¹ by 32% at 14 WAI while 80 t ha⁻¹ with the value of 24.28 mg kg⁻¹ resulted in least increase of 21% relative to control. But in comparison with the corresponding rates at 10 WAI, soil NO₃ ⁻–N decreased, Table 8. At 18 WAI, soil applied with cattle manure at the rate of 120 t ha⁻¹ gave NO₃ ⁻–N value of 58.28 mg kg⁻¹ by 106%, but 5 t ha⁻¹ with NO₃ ⁻–N value of 31.4 mg kg⁻¹ recorded the least increase of 11%, relative to control. Consequently, at 20 WAI, soil applied with NPK had NO₃ ⁻–N value of 43.28 mg kg⁻¹ and recorded the greatest increment in soil NO₃ ⁻–N by 116% followed by 80 t ha⁻¹ with NO₃ ⁻–N value of 40.00 mg kg⁻¹ that had 100% increases, while 10 t ha⁻¹ with soil NO₃ ⁻–N value of 29.24 mg kg⁻¹ had the lowest increase of 44% more than control. There was a decrease in soil NO₃ ⁻–N for cattle treated soil at 20 WAI compared to the corresponding rates at 18 WAI. However, at 22 WAI, soil appiled with cattle manure rates of 10 and 60 t ha⁻¹ with the same NO₃ ⁻–N value of 6.58 mg kg⁻¹ gave the highest increase of 107% while 150 t ha⁻¹ with NO₃ ⁻–N value of 3.31 mg kg⁻¹ gave the least increase of 4% in comparison with control. The soil NO₃ ⁻–N decreased in relation to the corresponding rates at 20 WAI.

Table 8 Influence of cattle manure rates and NPK fertilizer (t ha⁻¹) on mean soil NO₃ ⁻–N (mg kg⁻¹) at 2 weeks intervals

Effect of goat manure on soil NO₃ ⁻–N

At 2 WAI, soil applied with goat manure rate of 150 t ha⁻¹ had NO₃ ⁻–N value of 50.29 mg kg⁻¹ which recorded the highest increase of 133% while 5 t ha⁻¹ with soil NO₃ ⁻–N value of 23.13 mg kg⁻¹ recorded the least increase of 7% relative to control. At 4 WAI, the trend was in line with cattle manure treated soil as NH₄ ⁺–N. However, there were increases of soil NO₃ ⁻–N compared with the corresponding rates at 2 WAI. At 8 WAI, goat manure rate of 40 t ha⁻¹ with NO₃ ⁻–N value of 45.22 mg kg⁻¹ recorded the highest increase of 102% but 10 t ha⁻¹ with NO₃ ⁻–N value of 24.95 mg kg⁻¹ recorded the least increase of 11% above the control. In comparison of 8 WAI values with the corresponding rates at 4 WAI, soil NO₃ ⁻–N decreased. The soil with goat manure rate of 60 t ha⁻¹ which had NO₃ ⁻–N value of 398.54 mg kg⁻¹ recorded highest increase of 293%, 5 t ha⁻¹ gave NO₃ ⁻–N value of 227.02 mg kg⁻¹ with the least increase of 124% in relation to control at 10 WAI. However, at 12 WAI, 80 t ha⁻¹ resulted in soil NO₃ ⁻–N value of 137.85 mg kg⁻¹ and had highest increase of 51% but 5 t ha⁻¹ with soil NO₃ ⁻–N value of 100.70 mg kg⁻¹ had lowest increase of 11% more than the control. The result revealed that soil NO₃ ⁻–N decreased compared to the corresponding rates at 10 WAI. More so, at 14 WAI, 150 t ha⁻¹ with soil NO₃ ⁻–N value of 23.22 mg kg⁻¹ was significantly higher while other goat manure treated soil rates decreased in soil NO₃ ⁻–N compared with control. Consequently, the soil NO₃ ⁻–N decreased relative to corresponding rates at 12 WAI. The soil applied with goat manure at the rate of 150 t ha⁻¹ had NO₃ ⁻–N value of 82 mg kg⁻¹ and highest increase of 183%. The least increase of 4% was recorded from 5 t ha⁻¹ with soil NO₃ ⁻–N value of 30 mg kg⁻¹ compared with control, at 18 WAI, while 120 t ha⁻¹ with soil NO₃ ⁻–N value of 44.69 mg kg⁻¹ resulted in highest increment of 111% at 20 WAI. But 5 t ha⁻¹ gave 28.75 mg kg⁻¹ of NO₃ ⁻–N and recorded lowest increase of 36% in relation to control. In comparison with the corresponding rates at 18 WAI, there were decreases in soil NO₃ ⁻–N level. At 22 WAI, soil with goat manure rate of 120 t ha⁻¹ had NO₃ ⁻–N value of 5.79 mg kg⁻¹ and gave highest increase of 78% but 5 t ha⁻¹ with NO₃ ⁻–N value of 3.34 mg kg⁻¹ recorded least increase of 3% more than the control. The values also decreased compared to corresponding rates at 20 WAI, Table 9.

Table 9 Influence of goat manure rates and NPK fertilizer (t ha⁻¹) on mean soil NO₃ ⁻–N (mg kg⁻¹) at 2 weeks intervals

Effect of poultry manure on soil NO₃ ⁻–N

Table 10 presented that at 2 WAI, soil applied with poultry manure rate of 60 t ha⁻¹ had NO₃ ⁻–N value of 51.22 mg kg⁻¹ and recorded the highest increase of 127%. The rate of 10 t ha⁻¹ gave NO₃ ⁻–N value of 23.54 mg kg⁻¹ and recorded the least increase of 7% more than the control. The 4 WAI was also similar to others above. The soil with poultry manure rate of 120 t ha⁻¹ with the value of 35.42 mg kg⁻¹ recorded the highest increase of 59% in soil NO₃ ⁻–N while 20 t ha⁻¹ with NO₃ ⁻–N value of 23.74 mg kg⁻¹ had the least increase of 7% relative to control at 8 WAI. Soil NO₃ ⁻–N decreased in comparison with the corresponding rates at 4 WAI. Furthermore, soil applied with NPK gave highest NO₃ ⁻–N value of 261.76 mg kg⁻¹ with increment of 151% while 5 t ha⁻¹ with NO₃ ⁻–N value of 114.46 mg kg⁻¹ presented least increase of 10% compared with control at 10 WAI. However, soil with poultry manure rate of 80 t ha⁻¹ with 153.75 mg kg⁻¹ of NO₃ ⁻–N resulted to highest increment of 68% but 5 t ha⁻¹ with NO₃ ⁻–N value of 99.49 mg kg⁻¹ had lowest increase of 9% relative to control at 12 WAI. There were decrements at 12 WAI in relation to the corresponding rates at 10 WAI. More so, soil with poultry manure rate of 150 t ha⁻¹ recorded NO₃ ⁻–N value of 25.87 mg kg⁻¹ and gave the highest increase of 35% at 14 WAI. But soil NO₃ ⁻–N decreased in relation to the corresponding rates at 12 WAI. The rate of 120 t ha⁻¹ recorded highest NO₃ ⁻–N value of 46.55 mg kg⁻¹ by 62% and the least increase of 8% was recorded from 5 t ha⁻¹ with NO₃ ⁻–N value of 30.42 mg kg⁻¹ relative to control, at 18 WAI. Consequently, at 20 WAI, 80 t ha⁻¹ presented soil NO₃ ⁻–N value of 56.38 mg kg⁻¹ and this resulted in highest increment of 178% while 5 t ha⁻¹ had NO₃ ⁻–N value of 30 mg kg⁻¹ by 88% over the control. Increments in relation to 18 WAI were erratic. At 22 WAI, the rates of soil amended with poultry manure were not significantly different from control but NO₃ ⁻–N level reduced when compared to the corresponding rates at 20 WAI.

Table 10 Influence of poultry manure rates and NPK fertilizer (t ha⁻¹) on mean soil NO₃ ⁻–N (mg kg⁻¹) at 2 weeks intervals

Discussion

Soil characteristics

The soil used for the research was sandy clay loam (USDA 2010), slightly saline (LAS 2014), and slightly alkaline (Pam and Brian 2007) and these could support plant performances. More so, the soil was deficient in total nitrogen (McBride 2015; USDA-SCS 1974) and had low available P content (ENDMEMO 2015; Mallarino 2000) which could retard crop development. It contained moderate OC (McBride 2015; USDA-SCS 1974), exchangeable K⁺ and Mg²⁺ (Pam and Brian 2007); these were optimal for performance of the crop. The high exchangeable Na⁺ and Ca²⁺ contents (Pam and Brian 2007) could be attributed to the decomposition of the organic matter content of the soil due to high temperature, evaporation rate, sunshine hours/day and low rainfall during the research. This confirmed the studies of some researchers (Davidson and Jannssens 2006; Friedlingstein et al. 2006). The NH₄ ⁺–N and NO₃ ⁻–N contents were low and C: N ratio was considered normal (Hill 2001) and could be as a result of low rainfall during this period. The copper, iron and zinc contents were very high (Enwezor et al. 1989).

Manure characteristics

The EC of the three manures applied were very strongly saline (LAS 2014). This could be as a result of high concentration of cationic salts in the manures. More so, the highest EC value of poultry manure compared with the cattle and goat manures support the findings of Azeez and Van Averbeke (2010a, b) and could be attributed to the highest NH₄ ⁺–N, NO₃ ⁻–N and total P contents in poultry manure compared to others. The pH of the pure goat and poultry manures was considered mildy alkaline, whereas that of cattle manure was moderately alkaline (Pam and Brian 2007). This would allow for nutrients availability in the soil after mineralization. It was observed from the result that cattle manure had the highest total K, Na and Ca over the goat and poultry manures. However, goat manure was higher in total N, Mg and OC in relative to cattle and poultry manures. Very low value of total P in goat manure could be a result of the nutrient concentrations of the feed given to the animals. This is because they were fed with elephant grasses and wheat offal under intensive system although the nutrient contents of the feed were not measured. Equivalent amount of total N contents present in goat manure applied was higher than cattle and poultry manures. High P content in poultry manure could also contribute to the yield quality of crops at lower rates than cattle and goat manures. Cattle manure recorded highest equivalent amount of pH, total K, Na and Ca than goat and poultry manures; this could be attributed to the higher EC value in cattle manure than goat manure and confirmed the report of Monica (2013). Highest contents of total N, Mg and OC in goat manure compared with cattle and poultry manures suggested the reason for lowest value of EC in goat manure applied during the studies. This is because Mg was the only salt forming cation that was highest in goat manure. Highest total N could be a result of N level in the feed given to the goat animals, although the nutrient contents of the feed were not determined.

Manure mineralization processes in the soil

Incorporation of cattle, goat and poultry manures increased the concentration of NH₄ ⁺–N; this supports the findings of Sajal and Abul Kashem (2014) and NO₃ ⁻–N depending on the rates (Eghball et al. 2002) compared with control. However, the low values of NH₄ ⁺–N between 0 and 8 WAI of the manures could be attributed to conversion of NH₄ ⁺–N to NO₃ ⁻–N as supported in the study of (Hoskins 2015) that, between 2 and 4 weeks, NH₄ ⁺–N converted relatively quickly to NO₃ ⁻–N in a soil applied with broiler manure. However, mineralization of organic N into inorganic form of NH₄ ⁺–N and NO₃ ⁻–N occurred between 8 and 14 WAI of manures. Consequently, the low NH₄ ⁺–N between 18 and 22 WAI could be attributed to ammonia volatilization (Meissinger and Jokela 2000) while low NO₃ ⁻–N contents between 18 and 22 WAI could be attributed to leaching (Tom 2002; Marco et al. 2002) and plant root uptake. The low NO₃ ⁻–N contents in soil applied with NPK fertilizer at 18 WAI compared with 8 and 14 WAI could be attributed to acidic properties of NPK fertilizer, since acidity reduces soil microbial activity and this would in turn reduce nutrient mineralization (Xu et al. 2002).

Mineralization of NH₄ ⁺–N and NO₃ ⁻–N contents occurred in the soil with the maximum values at 10 WAI for cattle, goat and poultry manures but was contrary to the results of Azeez and Van Averbeke (2010b) that sharp increase in N release was at 120 days; and (Ayeni 2011; Ayeni and Adeleye 2011), that release of poultry manure nutrients proportion is between 1 and 2 months of incubation. The variability of mineralization processes could be attributed to microbial activities as suggested by Vinten et al. (2002); and climatic variations. Hence, the turnover processes and losses have influence on availability of nutrients in manures; this is why the prediction of manure nutrients to plants is problematic (Azeez and Van Averbeke 2010b; Sorenson 2001).

Very low N mineralization at 14 WAI compared to 12 WAI could be attributed to very low rainfall during this period. This corroborated the report of Jonathan (2006) that in dry soils, N mineralization is low because soil micro-organism activity is limited by water availability. More so, low NH₄ ⁺–N suggested to be as a result of volatilization of NH₃ as this is favored by warm temperature wet soils under drying conditions (Tom 2002; James 2001). While low NO₃ ⁻–N could be as a result of leaching that might have occurred at 12 WAI due to very high rainfall at this period, this also corroborated the work of Tom (2002) that NO₃ in the soil can be lost through percolation of water below the active root zone.

Across weeks after incorporation of the three manures, application rates of 20 t ha⁻¹ for cattle, 40 t ha⁻¹ for goat and poultry manure at the rate of 10 t ha⁻¹ were not widely significantly different when compared to higher rates. Therefore, addition of manures more than these rates to the soil could lead to wastage as these did not increase the amount of NH₄ ⁺–N and NO₃ ⁻–N in the soil sequentially according to this study. Although the high rates of cattle, goat and poultry manures were significantly higher relative to lower rates in this research, these suggested not to be encouraged as the increments were not regular across the weeks after manure application. The irregularity could be attributed to volatilization (Meissinger and Jokela 2000) and leaching (Tom 2002; Marco et al. 2002) of the NH₄ ⁺–N and NO₃ ⁻–N in the soil, respectively.

During this work, NPK fertilizer applied was able to supply equal amount of NH₄ ⁺–N to the soil when compared with 10 t ha⁻¹ of cattle manure treatment between 2 and 12 WAI. But goat manure treated soil at 60 t ha⁻¹ supplied equal amount of NH₄ ⁺–N compared to NPK fertilizer between 2 and 8 WAI, while 40 t ha⁻¹ of poultry manure was able to supply equal amount of NH₄ ⁺–N to the soil compared to NPK fertilizer at 0.4 t ha⁻¹ between 2 and 8 WAI. This suggested that cattle, goat and poultry manures incorporated to the soil at 10, 60 and 40 t ha⁻¹, respectively, supplied equal amount of NH₄ ⁺–N compared to NPK fertilizer at 0.4 t ha⁻¹ within 6 weeks. The low NH₄ ⁺–N and NO₃ ⁻–N contents of the soils applied with NPK fertilizer at 14 WAI compared with cattle, goat and manure treated soils rates could be attributed to acidic properties of NPK fertilizer. This corroborated the study results of Xu et al. (2002) that acidity reduces soil microbial activity resulting in reduced nutrient mineralization. This could also be attributed to depletion of soil organic matter as a result of synthetic fertilization (Jonathan 2006).

Conclusion and recommendation

Generally, addition of NPK 15–15–15 fertilizer, cattle, goat and poultry manures improved the soil NH₄ ⁺–N and NO₃ ⁻–N compared with control. The NH₄ ⁺–N and NO₃ ⁻–N contents of the soil between 2 and 8 WAI increased with the application of cattle, goat and poultry manures. However, between 8 and 14 WAI mineralization of NH₄ ⁺–N and NO₃ ⁻–N occurred but peak values were obtained at 10 WAI irrespective of the manures types and rates. Finally, the effect of cattle, goat and poultry manures on NH₄ ⁺–N and NO₃ ⁻–N decreased between 18 and 22 WAI.

To increase soil NH₄ ⁺–N and NO₃ ⁻–N for optimum production of crops yield, application of cattle, goat and poultry manures is recommended. It is recommended that application rate of 20 t ha⁻¹ for cattle, 40 t ha⁻¹ for goat and poultry manure at the rate of 10 t ha⁻¹ would provide optimum and improve the soil nutrients level which will be retained in the soil for longer period. To improve soil with low N contents, poultry manure is recommended. Meanwhile, addition of cattle, goat and poultry manures to supply NH₄ ⁺–N and NO₃ ⁻–N to the soil with regard to sowing dates is recommended to be 2 WAI as this would allow equilibration of the manures with the soil prior to planting, while incorporation of these three manures for crop needs is recommended to be 8 WAI.