Long-term effects of integrated soil fertility management practices on soil chemical properties in the Sahel

Geoderma 366 (2020) 114207

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Long-term effects of integrated soil fertility management practices on soil chemical properties in the Sahel

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Alexis M. Adamsa, Adam W. Gillespiea,b,c, Gurbir S. Dhillona, Gourango Kara, Colin Miniellya, Saidou Koalad, Badiori Ouattarae, Anthony A. Kimarof, Andre Bationog,1, Jeff J. Schoenaua, ⁎ Derek Peaka, a

Soil Science Department, University of Saskatchewan, 51 Campus Drive, Saskatoon, SK, S7N 5A8, Canada Canadian Light Source Inc., 44 Innovation Boulevard, Saskatoon, SK, S7N 2V3, Canada c Ontario Ministry of Agriculture, Food and Rural Affairs, 1 Stone Road West, Guelph, ON, N1G 4Y2, Canada d International Center for Tropical Agriculture (CIAT), P.O. Box 823-00621, Nairobi, Kenya e Institute of Environment and Agricultural Research (INERA), 04 BP: 8645, Rue Guisaga, Ouagadougou, Burkina Faso f World Agroforestry Centre (ICRAF), ICRAF-Tanzania Programme, P.O. Box 6226, Dar-es-Salaam, Tanzania g International Fertilizer Development Centre (IFDC), ICRISAT Sahelian Center BP 12404, Niamey, Niger b

ARTICLE INFO

ABSTRACT

Handling Editor: Karen Vancampenhout

Joint application of mineral and organic fertilizers and incorporation of legumes into cropping systems, known as integrated soil fertility management (ISFM), has improved short-term crop productivity in sub-Saharan Africa. Little research exists, however, on the effectiveness of long-term ISFM in improving soil quality and productivity. This study determined the long-term effects of different ISFM treatments on soil chemical properties and OM dynamics up to 20 cm soil depth at a long-term research site at Saria, Burkina Faso. The ISFM treatments applied from 1960 to 2008 included broadcasted fertilizer (100 kg ha−1 14-23-14 (NPK) with 50 kg ha−1 urea; and NPK with an additional 50 kg ha−1 urea and 50 kg ha−1 KCl) supplemented with crop residue retention, and with manure application at 5000 or 40000 kg ha−1. In addition, continuous cropping of Sorghum bicolor (sorghum) was compared to yearly rotation between sorghum and Vigna unguiculata (cowpea). The large manure rate (40,000 kg ha−1) supplement was most effective in buffering fertilizer-application-induced pH decline and increasing grain yield, soil carbon (C), nitrogen (N), and phosphorus (P) concentrations (p < 0.05). Manure application also enhanced the microbial cycling and retention of C and N microbial byproducts compared to other fertilizer treatments, as indicated by C and N X-ray Absorption Near Edge Structure (XANES) spectroscopies. Legume-cereal cropping led to increased abundance of C and N functional groups indicative of reduced OM breakdown compared to the continuous cropping system. Supplemental application of manure with mineral fertilizers under mixed cereal-legume cropping was found to be most effective in improving long-term soil fertility and crop productivity in the Sahel.

Keywords: Integrated Soil Fertility Management Long-term sustainability Sahel Soil fertility Soil organic matter dynamics XANES NEXAFS

1. Introduction Crop productivity levels in sub-Saharan Africa are less than half of the global average, largely due to low fertilizer use and soil nutrient depletion (FAO, 2014). The soil fertility and crop production issues in greater sub-Saharan Africa and the Sahel region are exacerbated by the arid climate and unreliable rainfall patterns (Saïdou et al., 2004). While soil fertility in the past has been managed through shifting cultivation and expanding cropping area, these practices are unsustainable with increased population pressure (Aune and Bationo, 2008). Since there is

little land to expand cropping, soil fertility must be managed through sustainably intensifying the use of crop inputs, especially fertilizers. Inorganic fertilizer is an important input to meet soil nutrient requirements, but small-scale Sahelian producers are typically not able to access or afford sufficient quantities of inorganic fertilizer because of poorly developed infrastructure, limited access to financing, and a weak private sector (Vanlauwe et al., 2010). Additionally, ammonification of nitrogen (N) fertilizers increases soil acidity and aluminum toxicity, which may lead to yield decline (Hue et al., 1991). Alternatively, applying organic fertilizers, such as crop residue or manure, will

Corresponding author. E-mail address: [email protected] (D. Peak). 1 Present Address: Action for Integrated Rural Development (AFIRD), KA98 46 G3121, Accra, Ghana. ⁎

https://doi.org/10.1016/j.geoderma.2020.114207 Received 29 May 2019; Received in revised form 3 December 2019; Accepted 19 January 2020 0016-7061/ © 2020 Elsevier B.V. All rights reserved.

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potentially buffer soil pH and increase soil C (Bado et al., 2012; Kibunja et al., 2012). As the quantity of organic amendments needed to meet crop nutrient demands are greater than what Sahel producers generally can obtain (Bationo and Mokwunye, 1991; Bayu et al., 2004), pairing organic and inorganic inputs is the best way for small-scale producers to meet crop requirements (Vanlauwe et al., 2010). Using a combination of soil fertility management practices, known as integrated soil fertility management (ISFM), is a practical way for producers to meet crop nutrient demand and build soil fertility (Vanlauwe et al., 2010). The goal of ISFM is to maximize nutrient-use efficiency and boost crop productivity, and practices in the ISFM approach include joint application of inorganic and organic fertilizers and incorporation of Nfixing legumes into cropping systems (Vanlauwe et al., 2010). Shortterm research on the effects of ISFM practices on soil fertility in the Sahel has been essential for developing the ISFM technique, and results show potential for ISFM to boost soil fertility and crop production (Geiger et al., 1992; Yamoah et al., 2002; Mando et al., 2005; Akponikpe et al., 2008; Chivenge et al., 2011; Gentile et al., 2013). Along with short-term studies, longer-term research is needed to understand how soil management practices influence soil fertility after several years of ISFM implementation (Reynolds et al., 2014). An assessment of the sustainability of reduced fertilizer rates, termed microdosing, was recently conducted by using soils from a longterm research trial under continuous Pennisetum glaucum (millet) in Sadore, Niger (Adams et al., 2016). Clear evidence existed of soil acidification and changes in C and N chemistry in soil organic matter of Sadore soils after application of 30 kg N ha−1 and 13.2 kg P ha−1 respectively, applied per year for 16 years. Although the soil acidification was less severe for reduced fertilizer rates compared to the recommended fertilizer rates, the entire system was experiencing reduced yields and was unable to accumulate SOM under reduced fertilizer rates, which suggested that some key aspects of ISFM were absent in the trial. The soils analyzed in the current research study, taken from trials initiated 50 years ago in Burkina Faso, are particularly valuable in revealing long-term effects of ISFM on soil fertility. The purpose of this research study was to provide scientific support for recommendations to small-scale farmers in the Sahel to both improve current soil fertility and sustain soil productivity for future generations. The specific objectives of this study were to determine the long-term effects of ISFM techniques, including mineral fertilizer, manure and crop residue application, and mixed cropping with legumes, on soil fertility and C and N cycling at the Saria long-term research site in Sahelian West Africa. It was hypothesized that ISFM treatments would increase soil fertility and nutrient cycling compared to fertilizer alone, which was tested through analysis of soil chemical properties and C and N K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy. XANES spectroscopy was expected to reveal the changes in C and N molecular speciation among the different ISFM treatments.

Table 1 Yearly addition of N, P, and K by organic amendment type and application rate. Amendment Type

Frequency

Rate kg ha

Low manure Large Manure Sorghum residue

Biennial Biennial Biennial

N

P

K

90 720 10

10.5 84 6.2

150 1200 32

−1

5000 40,000 4800

(sorghum) and a yearly rotation between sorghum and Vigna unguiculata (cowpea). The fertility treatments were as follows: 1) unfertilized control taken from soil near fenceposts at the site (Control), 2) fertilizer alone with 14 kg N ha−1, 23 kg P ha−1, and 14 kg K ha−1 (NPK 14-23-14) that were broadcast two weeks after sowing, followed by 23 kg N ha−1 broadcast as urea applied 30 days after sowing in the continuous sorghum system only (Fert), 3) fertilizer with biennial sorghum straw application (Fert + Residue), with the rate of straw being the amount produced the previous year, which was approximately 4800 kg ha−1 based on average straw yields during the experiment, 4) fertilizer with a low farmyard manure rate treatment of 5000 kg ha−1 (Fert + LM) applied every 2 years, 5) fertilizer with additional N and K (Fert + NK); with 23 kg N ha−1 as urea for the continuous system only, and an application of 30 kg K ha−1 for both cropping systems, both broadcast 60 days after sowing, and 6) Fert + NK treatment with a high farmyard manure rate of 40,000 kg ha−1 (Fert + NK + HM) applied every 2 years. Nutrient additions with sorghum straw and manure by rate are summarized in Table 1. The manure used in this study contained approximately 1.4% Ca, 0.6% Mg, and 0.2% Na, with a C:N ratio of 14.2:1. In contrast, the C:N ratio of sorghum straw was 304:1. Organic amendments were applied before land preparation and were incorporated into the soil by tillage. 2.2. Soil sample collection and analyses Soil samples were collected from plots from the 0–20 cm depth using a hand-coring device, air-dried, and ground to pass through a 2mm sieve, packaged, and shipped to the University of Saskatchewan. Soil was also sampled from adjacent uncultivated land to compare to cropped cultivated land. Soil samples were analyzed for pH, organic C (OC via combustion), total phosphorus (P) and nitrogen (N), available P, and sum of exchangeable base cations (Ca2+, Mg2+, Na+, and K+; SEBC). Soil pH was measured in triplicate using a glass electrode in a 2:1 (water volume: soil mass) suspension (Hendershot et al., 2008). The LECO-C632 C analyzer (LECO© Corporation, 1987) was used to analyze two 0.3-g replicates of each soil sample for OC concentration. Total N and P were measured according to the acid-block-digestion method of Thomas et al. (2010). Extractable and available P and sum of exchangeable base cations (Ca2+, Mg2+, Na+, and K+; SEBC) were determined using a Mehlich-3 extraction as described by Hendershot et al. (2008). The Mehlich-3 extraction was chosen because of its ability to extract multiple elements, and its applicability to tropical, acidic soils (Ziadi and Tran, 2008). Sum of exchangeable base cations (SEBC) was estimated by measuring the concentrations of Ca2+, Mg2+, Na+, and K+ in vacuum filtered Mehlich-3 solutions on the Microwave Plasma Atomic Absorption Spectrometer (MP-AES 4100, Agilent Technologies) and concentrations were used to calculate the sum of exchangeable Ca2+, Mg2+, Na+, and K+. Bulk density was not determined on a persample basis due to limited quantities of soil, but a previous studies on this site by Ouattara et al. (2006) showed that bulk density did not vary much as a function of treatment or replicate. In this study, the addition of cattle manure (40,000 kg ha−1), mineral fertilizer (100 kg ha−1 of NPK + 50 kg ha−1 of urea) and sorghum straw did not significantly change the bulk density compared to control soils, which ranged from 1.53 to 1.59 g cm−3 for 0 to 20 cm soil depth for different treatments on this experimental site (Ouattara et al., 2006). Thus, while the bulk

2. Materials and methods 2.1. Site description This long-term field trial was established in 1960 at the Environment and Agricultural Research Institute (INERA) organization in Saria, Burkina Faso (12°16′N, 02°09′W). Annual precipitation from 1960 to 2008 was 735 to 876 mm yr−1 and mean annual temperature ranged from 27.5 to 28.2 °C. The site’s soil is a Plinthic Lixisol (Oxic Alfisol), a slightly acidic soil with a kaolinite-rich B horizon, and is low in nutrient retention (Jones et al., 2013). Soil particle-size distribution is 53% sand, 36% silt, and 11% clay. The long-term trial was a split-plot design, with six fertility treatments as the main plot and cropping system as the split-plot. There were six replications for each treatment and cropping system combination. The two cropping systems included continuous Sorghum bicolor 2

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Fig. 1. Average sorghum grain yields for the integrated soil fertility management (ISFM) treatments during the years 1960–2008. Fert = fertilizer, LM = low manure, HM = high manure, fert + NK = fertilizer with additional N and K. Grain yields for the Fert + Residue treatment have been averaged for the years 2002–2008. The error bars represent standard error.

density of soil samples was not measured in the present study, it is assumed that the variation in bulk density between different treatments was minimal. Due to the lack of per-treatment bulk density, nutrient amounts were reported in concentration rather than content.

using fityk software package (ver. 1.2.1; Wojdyr, 2010) by fitting a series of Gaussian curves corresponding to 1 s-π* and 1 s-σ* spectral transitions, as well as a background arctangent curve corresponding to the ionization step, following the procedure outlined in Dhillon et al. (2017). XANES spectral features were assigned from diagnostic peaks, identified from analysis of reference compounds in previous work (Leinweber et al., 2010; Myneni, 2002; Urquhart and Ade, 2002) as follows: N K-edge spectra assignments were: (1) pyridinic-N at 398.7 eV, (2) amide-N (i.e., protein N) at 401.2 eV, (3) pyrrolic-N at 402.5 eV, (4) N-bonded aromatics including nitroaromatics at 403.5 eV, and aromatic amines at 404.5 eV (5) inorganic nitrate at 405.3 eV and (6) alpha amino-N at 406 eV. Carbon spectra assignments were: (1) aromatic C at 285 eV; (2) ketone at 286.6 eV; (3) phenolic at 287.1 eV; (4) carboxylic at 288.6 eV; and (5) carbohydrate hydroxyl at 289.6 eV. The peak heights of Gaussian curves corresponding to 1 s-π* spectral transitions were used to compare the relative abundance of C and N functional groups among different treatments.

2.3. X-ray absorption spectroscopy Carbon and N speciation was determined by X-ray Absorption Near Edge Structure (XANES) spectroscopy at the C and N K-edges using the spherical grating monochromator (SGM) beamline 11ID-1 at the Canadian Light Source in Saskatoon, Saskatchewan, Canada (Regier et al., 2007). The energy range for the C K-edge is from 270 to 320 eV, and the N K-edge is between 380 and 430 eV. Samples were prepared by making a slurry through addition of ~0.1 g of the soil sample to deionized water, pipetting onto Au-coated Si wafers attached to the sample holder using double-sided carbon tape, and allowing to air dry at room temperature. One replicate for each treatment was selected for XANES analysis. Next, samples were loaded into the SGM end station, and brought under vacuum, after which data were collected separately for C and N using the slew-scanning mode. In this technique, the monochromator scans the energy range of each element (Gillespie et al., 2015). Approximately 60 scans were taken per sample at a new spot on the sample for each scan to minimize radiation damage to the sample. The beam line exit slit was set to 25 μm and partial fluorescence yield was collected using one Amptek silicon drift detector. The C1s(C = O) → π*C=O transition at 288.8 eV of a citric acid standard was used for C K-edge energy calibration. Normalization to incident flux (I0) was carried out by recording the scattering intensity from a C-free and freshly sputtered Au surface across the C K-edge (Gillespie et al., 2015). The N K-edge data were calibrated to the υ = 0 vibration of interstitial N2 gas (at 400.8 eV) in solid-state ammonium sulfate (Gillespie et al., 2008). The pre-edge region of all spectra was subtracted to remove background, and all spectra were normalized to an edge step of one. The C and N XANES spectra were deconvoluted

2.4. Statistical analyses Mean comparisons of soil properties were performed with PROC MIXED in SAS (Version 9.4; SAS Institute, Cary, NC). The Tukey-Kramer test method of multi-treatment comparison for least significant differences (LSD) was used for mean separation. The fertilizer and crop rotation treatments were analyzed as randomized complete block design (RCBD) using 2-factor analysis of variance (ANOVA), with fertilizer and crop rotation treatments as the main effects and replications as the block effect. The interaction between fertilizer and crop rotations was also tested for all variables and found to be not statistically significant (p > 0.05). The grain yield data collected annually between 1960 and 2008 were analyzed using a linear model to calculate the regression of yield on time. Significance for yield and soil property means comparison was declared at p = 0.05.

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Fig. 2. a) Sorghum grain yield trends over time (1960–2008) for the integrated soil fertility management (ISFM) treatments. Only the regression for Fert + NK + HM was statistically significant, while the others were not significant at p < 0.05. b) Mean sorghum grain yield for the ISFM treatments compared to the mean yearly grain yield between 1960 and 2008.

3. Results and discussion

(1960–2008) was greater (p < 0.05) for the treatments with supplemental manure applications, followed by the treatments with mineral fertilizer and residue applications (Fig. 1). The application of mineral fertilizers increased the average grain yield by 1.4 times compared to the control, but additional increase in the rate of mineral fertilizer

3.1. Yield average and trends Sorghum grain yield averaged over the 48-year duration 4

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Fig. 3. Comparison of grain yield for the continuous sorghum and sorghum cowpea crop rotations for the integrated soil fertility management (ISFM) treatments between 2000 and 2008. Star (*) indicates statistically significant difference between the crop rotation within the ISFM treatment. The error bars represent standard error.

rotation systems, SEBC was found to be 3.8 cmolc kg−1 for the large manure treatment, and ranging from 0.8 to 2.1 cmolc kg−1 for all other treatments (Table 2). Application of fertilizer had an acidifying effect, which has also been reported previously (Bado et al., 2012; Kibunja et al., 2012; Adams et al., 2016). The crop residue application (4800 kg ha−1) was not enough to significantly increase pH or SEBC from the fertilizer-alone treatment. In other long-term Sahel research, soil pH increased with crop residue rates between 1600 and 4000 kg ha−1 depending on mineral fertilizer application rates (Geiger et al., 1992; Kretzschmar et al., 1991), and both soil pH and cation exchange capacity (CEC) were increased when 10 Mg ha−1 of millet straw were applied (de Ridder and van Keulen, 1990). More N fertilizer was applied at the Saria site (37–60 kg N ha−1) than in the recent analysis of the Sadore site (0–30 kg N ha−1), which may be why crop residue did not buffer pH decline from fertilizer application at the Saria site. Manure buffered the soil pH decline from fertilizer application, and pH buffering was increased with the large (40,000 kg ha−1) manure rate compared to the low (5000 kg ha−1) rate (Table 2). Likewise, SEBC increased with manure application, especially with the large manure rate compared to other treatments and control. Other long-term tropical research has shown that manure application rates greater than 5000 kg ha−1 are required to increase soil pH (de Ridder and Van Keulen, 1990; Eche et al., 2013; Manna et al., 2005). Greater manure rates of up to 40,000 kg ha−1 appear necessary to increase both soil pH and CEC (Cai et al., 2015; de Ridder and Van Keulen, 1990).

application did not lead to a significant increase in grain yields. In contrast, the addition of manure to fertilizer application at 40,000 kg ha−1 and 5000 kg ha−1 rates increased the grain yield by 8.7 and 3.6 times compared to the control, respectively. Sorghum grain yield for the large manure rate application (40,000 kg ha−1) showed a positive trend with a significant increase in grain yield over time (p = 0.018; Fig. 2a), while no significant relationship between yield and time was observed for the other fertilizer treatments. Stability analysis of grain yield for fertilizer treatments against yearly grain yields showed that the increase in grain yield for fertilizer treatments was resilient towards changing weather patterns over the years 1960 to 2008 (Fig. 2b). The positive yield trend over time with the large manure rate application suggests that the increased yield returns are not accompanied with a decrease in soil fertility and nutrient status, which has generally been observed in other long-term fertilizer application studies in Africa (Adams et al., 2016; Kibunja et al., 2012). The incorporation of legumes in the crop rotation also led to an increase (p < 0.05) in grain yield ranging from 34 to 181% for all fertilizer treatments (Fig. 3). Other long-term trials in West Africa have also reported an increase in crop yields for cereal-legume rotations compared to continuous cropping systems (Bado et al., 2012; Bationo et al., 2012). Incorporation of legumes into a crop rotation may increase crop productivity by adding biologically fixed N to the soil, as well as by improving other soil properties, such as nutrient availability and microbial biomass (Alvey et al., 2001; Formowitz et al., 2007). 3.2. Differences in soil pH and base cations with fertility treatments

3.3. Differences in SOC content with fertility treatments

Treatments receiving fertilizer and residue were all acidic, with soil pH ranging from 4.3 to 4.6, compared to the uncultivated and control soils, which had a soil pH of 5.3 and 5.6, respectively, when averaged over both crop rotation systems (Table 2). SEBC was very low for all treatments except the large manure treatment. Averaged over both crop

The low manure rate (5000 kg ha−1) and residue treatments did not significantly differ in SOC concentration from treatments with fertilizer alone (Table 2). Sandy soil texture and high year-around air temperatures at this site may be limiting the accumulation of SOC (Gentile

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Table 2 Soil chemical properties (0–20 cm depth) for the fertilizer and organic matter treatments averaged for the continuous sorghum and sorghum-cowpea rotation systems. Treatment

Control Fertilizer Fert + Residue Fert + LM Fert + NK Fert + NK + HM Uncultivated/uncropped Treatment Crop Rotation Treatment*Rotation

pH

5.6ba 4.3d 4.6d 5.0c 4.3d 6.4a 5.3bc < 0.0001 0.0576 0.4067

SEBC

Organic C

Total P

Available P

Total N

cmolc kg−1

%

mg kg−1

mg kg−1

mg kg−1

2.1b 1.5d 1.5 cd 2.0bc 1.5d 3.8a 0.8e < 0.0001 0.0968 0.5388

0.33b 0.38b 0.34b 0.45ab 0.38b 0.69a 0.32b 0.0004 0.2148 0.1173

98.5c 162.9b 124.2bc 169.1b 155.9b 279.3a 81.4c < 0.0001 < 0.0001 0.4135

7.7c 52.1b 34.5bc 52.6b 44.6b 137.6a 4.4c < 0.0001 0.0002 0.3316

124.0d 152.8bcd 131.6 cd 200.8bc 169.6bcd 466.3a 223.6b 0.0715 0.2224 0.1978

a

Means within a column followed by the same letter are not different (P > 0.05) using Tukey test for LSD. Fert = fertilizer; LM = low manure; fert + NK = fertilizer plus additional nitrogen and potassium; HM = high manure.

et al., 2013). Contrary to other fertilizer treatments, large manure rate (40,000 kg ha−1) addition increased SOC concentration significantly (Table 2). At long-term trials in more humid regions of Africa with similar soil texture, manure applied at 3000 to 10,000 kg ha−1 increased SOC compared to fertilizer alone (Bado et al., 2012; Janssen et al., 2011; Kibunja et al., 2012). The potential gains in SOC are likely to be lower in the Sahel than these other regions because the hot, dry climate leads to lower biomass input and increase OC decomposition (Yamoah et al., 2002; Janssen, 2011; Eche et al., 2013). In other long-term Sahelian research, manure at 6000 kg ha−1 increased SOC, indicating that the large manure rate adds enough additional C to compensate for SOC decomposition related to climate and soil texture in the Sahel (Nakamura et al., 2012). It is possible that some of the increase in SOC concentration due to large manure addition is offset by the potential reduction in soil bulk density due to increased OM content and porosity. However, other similar studies conducted at this experimental site, including Ouattara et al. (2006), did not observe a significant reduction in soil bulk density due to manure addition. Thus, the variation in soil bulk density due to the addition of manure has been assumed to be minimal in this study. Retention of crop residues did not improve OC compared to residue removal in this study as well as other long-term research in the Sahel (Buerkert and Lamers, 1999; Yamoah et al., 2002). Lack of OC improvement may be because crop residue rates were not large enough to increase soil C (Bationo and Buerkert, 2001) or alternatively because the large C:N ratio slowed decomposition of the residue to SOC (Ouédraogo et al., 2007). Manure application at rates exceeding 5000 kg ha−1, rather than the crop residue application, appear to be a better option to maintain and improve soil organic matter concentration in the Sahel.

Table 3 Total and available P and total N for cropping systems in surface (0–20 cm) soil averaged by the fertilizer treatments.

Site Saria

Cropping System Continuous sorghum Sorghum-cowpea rotation

Total P

Available P

Total N

mg kg−1 172.3aa 134.8b

mg kg−1 46.9a 30.7b

mg kg−1 208.7a 207.1a

a Means in a column followed by the same letter are not different at P > 0.05 according to Tukey’s LSD test.

manure rates at 10,000 to 17,000 kg ha−1 but not at 5000 to 6000 kg ha−1 in other Sub-Saharan Africa research (Kihanda and Warren, 2012; Zingore et al., 2007). Crop residue did not increase available soil P concentration compared to a control treatment in some research (Knewtson et al., 2008; Yamoah et al., 2002), but did increase available soil P in other studies (Geiger et al., 1992; Kretzschmar et al., 1991). It should also be noted that the soil properties, including soil P concentrations, were measured up to 20 cm soil depth in this study. Hence, it is possible that larger differences between treatments existed at shallower depths which were not sampled. The total and available soil P concentrations were lower by 22% and 46% (p < 0.05) respectively, in the mixed system compared to the continuous cereal cropping system averaged across fertilizer treatments (Table 3). Total and available soil P after several years may be greater with continuous cereal because legumes generally have greater P requirement than cereal crops (Knewtson et al., 2008). When introducing legumes into a crop rotation, small-scale producers should increase fertilizer or manure additions to maintain soil P.

3.4. Differences in total and available soil P with fertility treatments and cropping system

3.5. Differences in total soil N with different fertility treatments and under cultivation

The large manure rate of 40,000 kg ha−1 led to greater total and available soil P concentration compared to all other treatments (Table 2). The low manure rate and crop residue treatments did not lead to greater total or available soil P compared with fertilizer alone despite adding 10.5 kg ha−1 more P every year. Total P may not be increasing with the low manure and residue treatments because the additional P added with the OM was mineralized and taken up by plants along with fertilizer to meet plant-P demand (Reddy et al., 2000). The same demand-induced mobilization of crop residue P has been noted in other Sahel research (Hafner et al., 1993). Application rate may be important for available P, as an increase in available P was seen with

Only the fertilizers treatments incorporating manure inputs (i.e., fert + NK + HM and fert + LM) and the uncultivated soil treatment were significantly greater in total N concentration than the control treatment (Table 2). Other researchers have noted an increase in total N with 5000 kg ha−1 of added manure (Bationo and Mokwunye, 1991; de Rouw and Rajot, 2004). Crop residue at 2000 kg ha−1 did not increase total N in the acid sandy soils of West African Sahel (Buerkert and Lamers, 1999). The lack of increase in total N with the fertilizer-alone and fertilizer-with-residue-amendment treatments may be because

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Fig. 4. Normalized fluorescence yield of N K-Edge Xray absorption near-edge structure (XANES) spectra of surface soils in integrated soil fertility management (ISFM) treatments for each cropping system and adjacent uncropped/uncultivated soil. Continuous sorghum is the dotted line and sorghum-cowpea rotation is the solid line. Fert = fertilizer, LM = low manure, HM = high manure, fert + NK = fertilizer with additional N and K. Nitrogen features corresponding to specific excitation energy are identified as: 1. pyridines and pyrazines, aromatic N in 6membered rings at 398.8 eV; 2. amides at 401.2 eV; 3. pyrrolic, N in 5-membered rings with unpaired electrons, at 402.5 eV; 4. N-bonded aromatics at 403.5–403.8 eV; 5. nitrate-N at 405.3 eV; 6. alkyl-N at 406 eV.

fertilizer and organic amendments increase crop growth and subsequent N uptake and removal so that additional N does not build up in soil. Manure and crop residues add micronutrients and base cations to the soil in addition to N and P; for example, the low manure rate added 100, 70, and 30 kg ha−1 of K+, Ca2+, and Mg2+ respectively, based on the manure nutrient concentration analysis. Addition of these micronutrients, when the plant-N requirement is met will further improve yield and N uptake, which has been noted in other work (de Ridder and van Keulen, 1990; Geiger et al., 1992; Srivastava et al., 2002). Additionally, organic N may not be accumulating with lower-rate organic input treatments because the organic matter is not stabilized and protected in Sahel soils due to the sandy soil texture and elevated yearround air temperature, and instead is easily mineralized and taken up by plants or lost from the system (Gentile et al., 2013). Only the fert + NK + HM treatment receiving 37 kg N ha−1 as inorganic fertilizer and 720 kg N ha−1 as manure had greater total N concentration than the uncultivated soil. The uncropped and uncultivated soil likely had greater N because N in the cultivated land is exported with crop harvests and tillage stimulates microbial N mineralization. Similarly, in the absence of organic inputs, total soil N decreased over 25 years of continuous cultivation in Nigeria (Jaieyoba, 2003), and no- or reduced-tillage had greater total N than with tillage in the Burkina Faso region (Mando et al., 2005). However, there was no difference in OC between the cultivated cropped soil and uncropped

uncultivated soil in the current study (Table 3). Lack of differences may be because SOC decomposition rates were already large due to the sandy soil texture and warm year-round air temperatures (Chivenge et al., 2011; Gentile et al., 2013). 3.6. Effect of ISFM treatments on C and N speciation Normalized XANES are graphed from samples collected at the N Kedge (Fig. 4) and C K-edge (Fig. 5). The relative abundance of N functional groups differed with organic and mineral fertilizer inputs (Fig. 6). In general, the treatments incorporating organic inputs (i.e., Fert + NK + HM, Fert + LM, and Fert + Residue) showed greater abundance of amide-N, heterocyclic, and nitroaromatic compounds compared to the treatments incorporating only inorganic fertilizer inputs (i.e., Fert + NK, Fertilizer; Fig. 6). Specifically, the fert + NK treatment had the lowest abundance of amide-N (Fig. 6B), heterocyclicN compounds including pyridinic-N (Fig. 6A), and pyrrolic-N (Fig. 6C), and N-bonded aromatic compounds, including nitroaromatic-N and aromatic amines (Fig. 6D and 6E), compared to all other fertilizer and control treatments. In contrast, the fert + NK + HM treatment had the largest abundance of amide-N, heterocyclic, and nitroaromatic compounds compared to other treatments. The relative abundance of inorganic nitrates (Fig. 6F) did not differ among the fertilizer treatments. Amide-, heterocyclic-, and nitroaromatic-N compounds are formed

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Fig. 5. Normalized fluorescence yield of C K-edge X-ray absorption near-edge structure (XANES) spectra of surface soils in integrated soil fertility management (ISFM) treatments and adjacent uncropped/uncultivated soil. Continuous sorghum is the dotted line and sorghum-cowpea rotation is the solid line. Fert = fertilizer, LM = low manure, HM = high manure, fert + NK = fertilizer with additional N and K. Carbon features corresponding to specific excitation energy are identified as: 1. aromatic C at 285 eV; 2. ketones at 286.8 eV; 3. phenolic at 287.1 eV; 4. carboxylic at 288.6 eV; 5. carbohydrate hydroxyl at 289.6 eV.

at different stages in the degradation sequence of organic residues. Amides are components of proteins and are broken down earlier in the degradation sequence because they are an easily accessed N source for microbes (Albrecht et al., 2015; Gillespie et al., 2014; Vairavamurthy and Wang, 2002). Pyridinic- and pyrrolic-N are heterocyclic plant-derived N compounds that are relatively resistant to degradation and generally increase in abundance with greater microbial processing of organic matter (Mahieu et al., 2000; Vairavamurthy and Wang, 2002). These heterocyclic N compounds are formed through microbial degradation of proteinaceous compounds and are considered to be potential indicators of the degree of microbial metabolism (Gillespie et al., 2014). Nitrogen-bonded aromatics are formed through abiotic incorporation of N into the aromatic-C structure (Davidson et al., 2003). Aromatic-C is derived from microbial degradation of plant products rich in lignin, and acts as a precursor for the formation of N-bonded aromatic compounds (Albrecht et al., 2015). The relatively low abundance of microbially derived N compounds for the fert + NK treatment indicates that increased rate of mineral N fertilization led to a reduction in microbial decomposition. This observation is further supported by the relative abundance of ketones for different treatments. Ketones

showed trends in abundance similar to the N microbial degradation products, with the largest relative abundance for the fert + NK + HM treatment, and the lowest abundance for the fert + NK treatment (Fig. 7B). Greater abundance of ketones indicates large microbial OM turnover because ketones are a product of microbial aromatic C metabolism (Gottschalk, 1986) as well as a product of microbial fatty-acid metabolism (Chan et al., 2009; Dent et al., 2004). Other C functional groups did not show substantial changes in the relative abundance among fertilizer treatments (Fig. 7). Fertilizer-induced acidification, attributed to the nitrification of fertilizer N, is the most likely factor reducing microbial activity and decomposition in the fert + NK treatment, since the soil pH was substantially lower in the treatments with larger mineral fertilizer input compared to the control and treatments with high manure input (Table 2). Other studies have similarly observed a reduction in microbial activity and decomposition with increased N inputs in different ecosystems (Janssens et al., 2010; Fog, 1988) and suggested the shifts in microbial community structure and enzyme synthesis (Treseder, 2008) and the effects of acidification induced by the nitrification process (Graham and Haynes, 2005) as the primary reasons for reduced

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Fig. 6. Relative absorbance intensities (arbitrary units [a.u.]) of different N functional groups for the integrated soil fertility management (ISFM) fertilizer treatments of continuous sorghum and sorghum cowpea rotations.

microbial decomposition. The application of supplemental organic manure inputs buffered the decline in soil pH due to N fertilizers (Table 2), thus alleviating the reduction in microbial activity due to acidification (Marschner and Noble, 2000; Naramabuye and Haynes, 2007). Manure has also been shown to increase the microbial biomass and change the microbial community structure compared to chemical fertilization (Peacock et al., 2001; Acea and Carballas, 1988). Additionally, the application of manure may also have enhanced the retention of microbial byproducts (Jiao et al., 2006; Whalen and Chang, 2002), thus leading to the accumulation of microbial byproducts for the treatments with high manure inputs. Retention of microbial byproducts by manure may be especially important in the sandy soils of Saria, since their fine fraction is small (~10%), and hence, not conducive to the retention of microbial byproducts (Gentile et al., 2013; Grandy and Neff, 2008). Among cropping systems, amide-N abundance was lower in the continuous cereal than legume rotation for each treatment (Fig. 6B). These results indicate amides are building up in the legume rotation compared to the continuous cereal system likely due to larger protein and amino acid concentrations in legume residues. Similarly, the phenol abundance was greater in the legume rotation than under continuous cereal for all treatments except the fert + residue treatment (Fig. 7C). Phenols are derived from plant materials and lignin, which are relatively resistant to breakdown (Gillespie et al., 2014; Grandy and Neff, 2008; Wickings et al., 2012), but may break down if the soil and conditions are suitable for mineralization (Gillespie et al., 2014).

Greater abundance and build-up of amides and phenols in the mixedcropping systems indicates that OM degradation is less in the mixedcropping system compared to the continuous-cereal system. 4. Conclusions In Sahelian soils, biennial applications of manure applied at large rate (40,000 kg ha−1) helped to improve soil C, N, and P concentrations. The C and N functional group abundance, as determined by XANES spectroscopic analysis, indicated that the addition of mineral-N fertilizers may be suppressing microbial activity and decomposition, perhaps due to fertilizer-induced soil acidification. Addition of manure buffered the decrease in soil pH from N fertilization, possibly ameliorating the decline in microbial activity. Addition of manure may also have facilitated the retention of microbial byproducts in sandy-textured soils of the Sahel region. An abundance of amides and phenols present in SOM under mixed legume-cereal cropping indicates lower levels of SOM degradation and a greater supply of labile OM in the legume rotation compared to the continuous cropping system. However, inclusion of legumes in cropping systems required greater soil P additions. Along with application of mineral fertilizer to meet nutrient requirements, increased OM input and integration of legumes with cereal cropping may sustainably improve soil productivity and food security in the Sahel region of Africa, as anticipated by ISFM management practices. While this study indicates a strong role of manure amendments in maintaining long-term soil fertility in the Sahel, previous studies in this

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Fig. 7. Relative absorbance intensities (arbitrary units [a.u.]) of different C functional groups for the integrated soil fertility management (ISFM) fertilizer treatments of continuous sorghum and sorghum cowpea rotations.

region have indicated limited availability of animal manure to the small-scale producers especially following the drought years (Williams et al., 1995). Thus, proper policy needs to be put in place to increase the ability of small-scale producers to increase livestock and manure production as well as access crop residue and manure sources for their land.

Appendix A. Supplementary data

Declaration of Competing Interest

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.geoderma.2020.114207. References

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements International Development Research Centre, the Government of Canada through Foreign Affairs, Trade and Development Canada, and the Natural Science and Engineering Research Council of Canada funded this research. Research described in this paper was performed at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research. Thank you also to Augustine Osei for assistance with laboratory research work, and to Drs. Ken Van Rees and David Natcher for research and writing suggestions.

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