Agriculture, Ecosystems and Environment

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1 Agriculture, Ecosystems and Environment 131 (2009) Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment journal homepage: Managing N availability and losses by combining fertilizer-n with different quality residues in Kenya R. Gentile a, *, B. Vanlauwe b, C. van Kessel a, J. Six a a Department of Plant Sciences, University of California, One Shields Ave., Davis, CA 95616, USA b Tropical Soil Biology and Fertility Institute-CIAT, P.O. Box 30677, Nairobi, Kenya ARTICLE INFO ABSTRACT Article history: Received 3 December 2008 Received in revised form 5 February 2009 Accepted 9 February 2009 Available online 10 March 2009 Keywords: Residue quality N fertilizers Mineralization Leaching Maize The integrated soil fertility management paradigm, currently advocated in Sub-Saharan Africa for rehabilitating its soils, recognizes the possible interactive benefits of combining organic residues with mineral fertilizer inputs on agroecosystem functioning. Residue quality may be a controlling factor for any beneficial interactions. The objectives of this study were to determine the effect of different quality organic residues and mineral fertilizer on N cycling under field conditions in Embu, Kenya. We hypothesized that combining low quality residue with mineral N would reduce potential system losses of N by synchronizing N release with plant uptake. Residue treatments consisted of a control (no residue input), high quality tithonia (Tithonia diversifolia) residue (C to N ratio of 13:1) and low quality maize (Zea mays) stover residue (C to N ratio of 42:1) applied at a rate of 1.2 Mg C ha 1. Subplots of each residue treatment received either 0 or 120 kg N ha 1 in a split-application, and maize was cultivated each season. During the 11th growing season of the trial (March September 2007), we monitored soil mineral N, potential gross mineralization and nitrification rates, and plant N content. Extractable mineral N in the soil profile varied with residue and fertilizer inputs throughout the growing season. The tithonia treatments showed early season N release of 22 kg N ha 1 in the upper 30 cm of the soil profile. The maize + fertilizer treatment displayed an immobilization of 34 kg N ha 1 after the application of N fertilizer. However, the lower mineral N of the maize + fertilizer treatment did not reduce crop N uptake, as mineral N in the other fertilizer treatments was leached from the upper soil (0 60 cm) at 57 d after planting. The interactive effect on crop yield and N uptake of combining residue with fertilizer-n changed from negative to positive as residue quality decreased. The benefit of combining low quality residue with N fertilizer in reducing N losses indicates that this soil fertility management strategy should be adopted in environments subject to high N leaching losses. ß 2009 Elsevier B.V. All rights reserved. 1. Introduction Soil fertility decline is seen as the most important constraint to crop production in Sub-Saharan Africa, where most agroecosystems remove more nutrients than are provided by external inputs (Sanchez and Jama, 2002). As mineral fertilizers and organic residues are often not available or affordable in sufficient quantities or qualities to be used alone, integrated soil fertility management (ISFM) is currently promoted as a management approach that optimizes the use of all available resources within each target environment (Kimani et al., 2003). The ISFM approach advocates the combined use of organic residues and mineral fertilizers, which may resolve the practical limitation of input availability, but which may also benefit crop N synchrony and N * Corresponding author. Present address: AgResearch, Tennent Drive, Private Bag 11008, Palmerston North, New Zealand. Tel.: ; fax: address: roberta.gentile@agresearch.co.nz (R. Gentile). loss reduction through interactive effects between both types of inputs. Vanlauwe et al. (2001a) developed a direct and indirect hypothesis to explain potential beneficial interactive effects of combining fertilizer and residue inputs. The direct hypothesis states that the temporary immobilization and subsequent release of fertilizer-applied N due to microbial decomposition of added residues may improve N synchrony. The concept of N synchrony is that the supply of available N from the soil can be matched in quantity and time with plant uptake requirements (Myers et al., 1994). The application of readily available fertilizer-n will create high levels of available N that exceed plant demand early in the season and can lead to potential N losses. Added residue slowly releases N, often after the peak of plant N demand has occurred, leading to N deficiencies (Myers et al., 1994). However, combining fertilizer with residue may serve to match the rate of soil N supply with the rate of plant N uptake. Synchronizing soil N availability with plant requirements can improve system N use efficiency and reduce N losses through leaching beyond the crop rooting depth or /$ see front matter ß 2009 Elsevier B.V. All rights reserved. doi: /j.agee

2 R. Gentile et al. / Agriculture, Ecosystems and Environment 131 (2009) gaseous emissions. In support of the direct hypothesis, incorporating maize (Zea mays) residue with N fertilizer reduced leaching of fertilizer-n in a lysimeter study (Vanlauwe et al., 2002). The indirect hypothesis for interactive effects of combining input types states that the application of residues in addition to fertilizer may improve soil conditions aside from the nutrients applied with the fertilizer (e.g. water holding capacity), which may result in higher demand by plants for the fertilizer nutrients and consequently enhance fertilizer nutrient use efficiency (Vanlauwe et al., 2001a). As evidence for the indirect hypothesis, Vanlauwe et al. (2001b) found that combining organic residues and fertilizer improved soil moisture conditions and thereby increased yields of maize during drought years. The hypothesized interactive benefits of combining fertilizer and residue inputs are predicted to be controlled by residue quality (Vanlauwe et al., 2001a). Residue N content is an important defining parameter for residue quality with high quality residues having greater N contents than low quality residues. As residue quality affects the balance of N mineralization and immobilization (Heal et al., 1997), it will change the quantity and timing of fertilizer-n immobilization and release, thereby altering the interactive effect of combining inputs. For example, in the lysimeter study that showed reduced N leaching when fertilizer was combined with maize residue, this reduction was minor when fertilizer was combined with high quality Mucuna pruriens residue (Vanlauwe et al., 2002). Sall et al. (2003), found patterns of C mineralization and N immobilization in a 60-d incubation were modified by residue quality when residues were combined with N fertilizer. Fertilizer additions stimulated early C mineralization and N immobilization with lower quality Casuarina equisetifolia, but inhibited these processes with higher quality Faidherbia albida residue. In a previous incubation study, we found that the interactive effect on available mineral N of combining fertilizer with residue changed from negative to positive with increasing residue quality (Gentile et al., 2008). Combining fertilizer with high quality residue both immobilized less fertilizer-n and stimulated the release of more residue-n than the combination of fertilizer with low quality residue. Given this influence of residue quality on the interactive effects of combining input types in the incubation study, this relationship needed to be tested under field conditions. Therefore, the objective of this study was to determine the effect of residue quality on the interactive effect of combining residue and fertilizer inputs on mineral N availability versus N loss under field conditions. We hypothesized that combining low quality residue with mineral N fertilizer would reduce system N losses and optimize plant N uptake, thereby enhancing the interactive effect of combining input types. with organic residue input as the main plot and N fertilizer application as the subplot treatments. There were three replicated blocks and subplots measured 6 m 5 m. The residue treatments consisted of a no-input control, tithonia (Tithonia diversifolia), and maize. The residues, consisting of pruned leaves of tithonia and stovers remaining after cob removal of maize, were applied at a rate of 1.2 Mg C ha 1 year 1. Nitrogen fertilizer, as calcium ammonium nitrate (5Ca(NO 3 ) 2 NH 4 NO 3 ), was applied at a rate of 0 or 120 kg N ha 1 growing season 1. All plots received basal fertilizer applications of 60 kg P ha 1 season 1 and 60 kg K ha 1 season 1. Maize was grown in all plots each season. During the 11th consecutive growing season of the trial, from March to September 2007, mineral N dynamics and crop production were monitored. On March 2007, the residues and basal P and K fertilizers were incorporated into the soil by broadcasting and hand hoeing to a depth of 0 15 cm. The tithonia residues were harvested on site from plants grown for the trial and the maize residues were from the previous season s harvest. Subsamples of each residue were analyzed for total C and N content to calculate the equivalent rate of biomass to apply 1.2 Mg C ha 1.Thetworesidue types differ in quality due to their N concentrations. The residue C to N ratio was 13:1 for tithonia and 42:1 for maize. At the onset of the rains on 4 April 2007, maize (hybrid 513) was seeded with a plant spacing of 25 cm 75 cm. The seeding rate was two plants per hole, and plants were thinned 26 d after planting (DAP) to one plant per hole. Mineral N fertilizer was applied to the subplots in a split application by broadcasting with light incorporation. At 30 DAP, 40 kg N ha 1 was applied and the remaining 80 kg N ha 1 was added at 54 DAP. The plots were weeded by hoeing and hand pulling periodically throughout the season. A meteorological station on site recorded daily precipitation (see Fig. 1) Soil sampling Soil samples were collected throughout the season (11 days before planting ( 11) and 15, 29, 49, 83, and 169 DAP) to measure extractable mineral N. Soil was sampled to a depth of 150 cm with an Edelman auger in increments of 0 15 cm, cm, cm, cm, cm, and cm. Composite samples for each depth increment were collected from four profiles per plot. Soil samples were stored in polyethylene bags, transported in coolers with icepacks from the field, and stored at 5 8C until extractable mineral N analyses were performed. For extractable mineral N analysis, a 20-g subsample of moist soil was extracted with 100 ml of 2 M KCl solution by shaking on a 2. Materials and methods 2.1. Site description and experimental design Our study was conducted at a long-term field experiment in Embu located in the central highlands of Kenya ( S, E; 1380 m a.s.l.). The site has a mean annual temperature of 20 8C and a mean annual rainfall of 1200 mm. Rainfall occurs in two distinct periods from March to June and October to December, which allow for two growing seasons per year. The soil is a red clay Humic Nitisol (FAO, 1998) dominated by kaolinite minerals. The texture of the surface soil (0 15 cm) is 17% sand, 18% silt, and 65% clay. At the start of the experiment, the surface soil contained 29.4 g kg 1 organic C and 2.7 g kg 1 total N. A long-term trial to evaluate the repeated application of different quality organic residues alone and in combination with N fertilizer on soil organic matter dynamics was established at the site in March The experiment consists of a split-plot design Fig. 1. Cumulative precipitation during the March September 2007 growing season at Embu, Kenya (DAP = days after planting).

3 310 R. Gentile et al. / Agriculture, Ecosystems and Environment 131 (2009) reciprocal shaker for 1 h and filtering through a Whatman No. 42 ashless filter paper. The NH + 4 -N and NO 3 -N concentrations of the soil extracts were analyzed colorimetrically by the Berthelot reaction for NH + 4 (Forster, 1995), and vanadium(iii) chloride reduction for NO 3 (Doane and Horwath, 2003). Total extractable N was calculated as the sum of NH + 4 -N and NO 3 -N concentrations. A second 20-g subsample was oven-dried at 105 8C to calculate the soil moisture content. Remaining soil from the 0 to 15 cm depth was sieved through a 2-mm sieve and air-dried for N transformation rate measurements. To express mineral N soil concentrations on an area basis, bulk density samples were collected for each depth increment with a 4.9 cm diam. 5 cm core. For the 0 15 cm increment, 2 cores were collected in each plot at a depth of 5 10 cm. For the lower depths, three pits were excavated to 150 cm depth at the field site and 2 cores per pit were collected at the middle of each depth increment (22.5, 45, 75, 105, and 135 cm). The mean bulk density values for each of the lower depths were used for all plots N transformation rates Potential gross mineralization and nitrification rates were measured in the 0 15 cm depth at each soil sampling time using 15 N pool dilution techniques (Barraclough, 1991). Briefly, four 20-g replicates of air-dried soil were weighed into 120-ml specimen cups and brought to 31% water holding capacity with distilled water. The soils were allowed to equilibrate for 2 d to avoid the period of stimulated microbial activity upon rewetting air-dried soil (Birch, 1958). To start the incubations, solutions of ( 15 NH 4 ) 2 SO 4 (99 atom%) for gross mineralization or K 15 NO 3 (99 atom%) for gross nitrification were added to two replicate samples per treatment to bring the soil to 55% water holding capacity. The quantity of 15 N added in solution was adjusted for each sampling time to achieve approximately 10 atom% 15 N-enrichment of the initial NH 4 + or NO 3 pool size. For the samples collected at 29 and 49 DAP, gross N transformation rates were mimicked for the period following N fertilizer applications. For these sampling times, nitrogen as NH 4 NO 3 was added to all N fertilizer treatments with the 15 N solution at rates equivalent to 40 kg N ha 1 for 29 DAP and 80 kg N ha 1 for 49 DAP. All samples were incubated at a constant temperature of 25 8C. The two replicate samples for each treatment were destructively sampled after 2 h (t = 0) and 2 d (t = 2). Samples were extracted with 2 M KCl, and the quantity of NH 4 + -N or NO 3 - N was measured as previously described. The 15 N isotopic signature of the NH 4 + -N or NO 3 -N was determined by diffusion onto acidified disks (Stark and Hart, 1996) and analyzed with a PDZ Europa isotope ratio mass spectrometer (Crewe, United Kingdom) at the Stable Isotope Facility of the University of California-Davis. Rates for gross mineralization were calculated using the following equation: logða m ¼ u 0 =A t Þ logðð1 þ utþ=c 0 Þ where m = gross mineralization rate, u = the rate of change in the size of the NH + 4 pool, A 0 ¼ the 15 N atom% excess of the NH þ 4 pool at t ¼0; A t ¼the 15 N atom% excess of the NH þ 4 pool at t ¼ 2, and C 0 = the size of the NH + 4 pool at t = 0. Rates for gross nitrification were calculated using the same equation but with 15 N enrichments and pool size changes for NO 3. Mineralization rates for 169 DAP were not calculated because of too low NH + 4 pool sizes Plant sampling Plant biomass samples were taken during the season (26, 49, 83, and 168 DAP) to measure maize production and N uptake. At 26 (1) DAP, 10 plants per plot were collected when thinning the plots to 1 plant per hole. At 49 and 83 DAP, 6 plants per plot were sampled by cutting plants at the base of the stalk at the soil surface. The final sampling (168 DAP) corresponded with maize harvest and a net plot area of 8.25 m 2 that had not been previously sampled was harvested. Maize biomass was separated into stover and ears and field weights of all above-ground biomass were recorded. Subsamples of the stover and ears were taken for moisture correction, and the ears were separated into grain and cob components. All plant samples were oven dried at 65 8C to a constant weight and ground. Plant tissues were analyzed for total N with a PDZ Europa isotope ratio mass spectrometer. The interactive effect (IE) on maize yield and N uptake of combining fertilizer and residue inputs for each residue quality was calculated using the following equation from Vanlauwe et al. (2001a): IE ¼ F þ R Con ðf ConÞ ðr ConÞ (2) where F + R = fertilizer + residue treatment, Con = control treatment with no inputs, F = fertilizer treatment, and R = residue treatment Statistical analyses Analyses of variance were performed on all measured variables using Proc MIXED in the SAS statistical software (SAS Institute, Cary, NC). Data were analyzed for a split-plot design with the main effects of residue, fertilizer, and residue fertilizer treated as fixed effects, and rep and rep residue considered as random effects. Effects were deemed to be significant at P < 0.05 with trends identified at P < 0.1. Treatment means were subsequently separated using the PDIFF option of the LSMEANS statement. Mineral N availability data were analyzed separately for each depth, except for the data from 0 to 15 cm and 15 to 30 cm, which were summed for analysis, because similar trends were observed for the two upper soil layers. Likewise, the data for the cm and cm depths showed the same pattern of treatment differences and were also combined. 3. Results 3.1. Mineral N dynamics Residue and N fertilizer inputs significantly altered the pattern of mineral N availability in the different depths of the soil profile during the growing season (Table 1). At the start of the season ( 11 DAP), the N fertilized treatments had higher levels of mineral N throughout the soil profile than the unfertilized treatments (Fig. 2). With the onset of the rains, this significant fertilizer effect disappeared from the upper 0 30 cm but remained in the lower depths for the duration of the season. After the addition of residue, the tithonia treatments showed early mineralization and higher levels of mineral N at 0 30 cm. There was an average of 17 and 22 kg N ha 1 more mineral N at this depth with tithonia residue compared to the control and maize treatments at 15 and 29 DAP, respectively. Additionally, this pulse of mineral N in the tithonia treatment tended to be greater in the lower cm and cm depths at 49 DAP (P < 0.1). Following the first fertilizer application, a significant residue fertilizer interaction was observed for the 0 30 cm depth. At 49 DAP, adding N fertilizer in the control and tithonia treatments significantly increased mineral N. However, no difference was observed with the addition of fertilizer in the maize treatment. The maize + fertilizer treatment had an average of 34 kg N ha 1 less mineral N than the control + fertilizer and tithonia + fertilizer treatments in the 0

4 R. Gentile et al. / Agriculture, Ecosystems and Environment 131 (2009) Table 1 Statistical significance of residue and fertilizer treatment effects on extractable mineral N in the cm soil profile at four depths and six sampling dates (DAP = days after planting) during the 2007 maize growing season. Depth (cm) Source of variation 11 DAP 15 DAP 29 DAP 49 DAP 83 DAP 169 DAP 0 30 Residue (R) ns ** ** ns ns ns Fertilizer (F) ** ns ns ** ** ** R F ns ns ns ** ns ns Residue (R) ns ns ns * ns ns Fertilizer (F) ** * * ** ** ** R F ns ns ns ns ns ns Residue (R) * ns ns * ns ns Fertilizer (F) ** ** ** ** ** ns R F * ns ns ns * ns Residue (R) ns ns ns ns ns ns Fertilizer (F) ** ** * ** ** ** R F ns ns ns ns ns ns ns = not significant. * Significance level indicated as P < 0.1. ** Significance level indicated as P < cm depth. By 83 DAP, mineral N levels showed a depletion relative to 49 DAP in all depths (Fig. 2), as this corresponded to the period of greatest crop N uptake (Fig. 3). There was a trend for a residue fertilizer interaction at the cm depth, with the control + fertilizer and tithonia + fertilizer treatments again showing higher mineral N levels than the other treatments. At the end of the growing season from 83 to 169 DAP, the fertilizer treatments were again showing higher levels of mineral N in all depths of the soil profile, except cm at 169 DAP N transformation rates No interactions between organic residue and fertilizer additions on N transformation rates were observed for any of the samplingdates (Table 2). Addingresidue increased potential rates of gross mineralization and nitrification on several sampling dates during the growing season (Fig. 4). Gross mineralization showed trends of higher rates with maize and tithonia additions than the control at 15 DAP, and were significantly greater with tithonia versus the other treatments at 49 DAP. Similarly, the two residue input treatments significantly increased gross nitrification rates as compared to the control by an average of 2.2 and 2.0 kg N ha 1 d 1 at 11 and 49 DAP, respectively. Inputs of N fertilizer, had no effect on gross mineralization rates, but significantly increased gross nitrification rates at 83 DAP (Table 2). At 83 DAP, gross nitrification rates were 2.84 kg N ha 1 d 1 in the unfertilized treatment and 7.29 kg N ha 1 d 1 with fertilizer (data not shown). Fig. 2. Extractable mineral N at four depths in the soil profile for different residue and fertilizer treatments at six sampling dates (DAP = days after planting) during the 2007 maize growing season. Error bars represent the standard error of the difference. Fig. 3. Crop N uptake rates for different residue and fertilizer treatments during the 2007 maize growing season (DAP = days after planting). Error bars represent the standard error of the difference.

5 312 R. Gentile et al. / Agriculture, Ecosystems and Environment 131 (2009) Table 2 Statistical significance of residue and fertilizer treatment effects on potential gross N transformation rates at six sampling times (DAP = days after planting) during the 2007 maize growing season. N transformation Source of variation 11 DAP 15 DAP 29 DAP 49 DAP 83 DAP 169 DAP Mineralization Residue (R) ns * ns ** ns Fertilizer (F) ns ns ns ns ns R F ns ns ns ns ns Nitrification Residue (R) ** ns ns ** ns ns Fertilizer (F) ns ns ns ns ** ns R F ns ns ns ns ns ns ns = not significant. * Significance level indicated as P < 0.1. ** Significance level indicated as P < was the only treatment to influence crop grain yields, with fertilizer additions yielding 1.45 t ha 1 more than without N fertilizer (Table 3). Total N uptake of the grain averaged 67 and 40 kg N ha 1 for the treatments with and without fertilizer additions, respectively. Residue quality influenced the yield interactive effect of combining residue with fertilizer inputs (Table 3). For the low quality maize residue, combinations with N fertilizer resulted in zero to a trend for positive interactive effects for grain yield and grain N uptake. Conversely, the high quality tithonia residue produced negative interactive effects. 4. Discussion 4.1. Residue quality influence on interactive effects Fig. 4. Potential gross mineralization and nitrification rates for different residue treatments during the 2007 maize growing season (DAP = days after planting). Error bars represent the standard error of the difference Crop production Maximum N uptake rates occurred prior to 83 DAP (i.e. flowering stage), as indicated by the majority of the maize N content taken up by that day (Fig. 3). This period of high plant N uptake, corresponded with the largest depletion of mineral N in the soil profile (Fig. 2). While residue input significantly interacted with fertilizer to influence upper soil mineral N levels during this period, it did not affect crop N uptake. Only N fertilizer had a significant effect on crop N uptake, with fertilizer additions leading to greater N uptake rates at 36 and 66 DAP. Similarly, N fertilizer Table 3 Maize grain yield and N uptake for different residue and fertilizer treatments, and the interactive effects of combining residue with fertilizer. Treatment Grain yield (t ha 1 ) Grain N (kg N ha 1 ) Control Control + fertilizer Maize Maize + fertilizer Tithonia Tithonia + fertilizer SED residue SED fertilizer Interactive effect Maize 0.38 ns 9.5 * Tithonia 0.48 ** 1.9 ** ns = not significant. * Significance level indicated as P < 0.1. ** Significance level indicated as P < Residue quality influenced soil mineral N dynamics during the growing season both when applied alone and in combination with mineral N fertilizer. In support of our hypothesis, low quality maize residue reduced N losses when combined with fertilizer and resulted in positive interactive effects on crop N uptake. In contrast, the high quality tithonia residue showed early season mineralization and N losses and resulted in negative interactive effects on crop N uptake. Residue quality controlled the balance of net mineralization versus net immobilization, which altered the amount of mineral N susceptible to leaching losses or plant uptake. According to the direct hypothesis for interactive benefits of combining residue and fertilizer inputs, the temporary immobilization of fertilizer-n and subsequent remineralization of N due to microbial decomposition of added residues may improve N synchrony (Vanlauwe et al., 2001a). Residue quality was observed to control the immobilization of fertilizer-n according to this hypothesis, which resulted in positive interactive crop effects for combining fertilizer with low quality maize residue, but not with high quality tithonia residue Residue quality influence on N dynamics The relationship between residue quality and N dynamics is illustrated by soil mineral N and gross N transformation rates during the growing season. With the application of high quality tithonia residue, an early increase in mineral N in the 0 30 cm depth was observed at the first two sampling times after residue incorporation. This increase indicates there was an immediate net mineralization with tithonia additions, resulting in an average release of 24% of the applied tithonia-n by 29 DAP. In contrast, while the incorporation of residues stimulated gross mineralization rates for both maize and tithonia at 15 DAP, there was no apparent net mineralization with the low quality maize residue. The tithonia residue had a low C to N ratio (13:1) and could, therefore, supply the majority of the N requirement for decom-

6 R. Gentile et al. / Agriculture, Ecosystems and Environment 131 (2009) posing microorganisms, thus eliminating any period of net N immobilization (Mary et al., 1996). Net N mineralization has been observed in 3 28 d in incubation studies with high N content residues (Sakala et al., 2000; Vanlauwe et al., 2005). However, the release of N from the tithonia residues occurred in advance of crop N uptake, which peaked at 66 DAP. By 26 DAP, maize plants had only accumulated 30 kg ha 1 in above-ground biomass and would have had shallow root systems (Weaver, 1926). Therefore, the early release of mineral N in the upper profile of the tithonia treatment was rapidly leached to the cm depths by 49 DAP, where it may have become unavailable for crop N uptake. Likewise, Mtambanengwe and Mapfumo (2006) observed increased mineral N in the topsoil at 21 d after incorporation of high quality Crotalaria juncea, but most of this N was leached down the soil profile below the rooting depth before crop N uptake Residue interactions with fertilizer The influence of residue quality on N dynamics was further noted by its interaction with N fertilizer. After the first application of fertilizer, a net immobilization was observed in the maize + fertilizer treatment, whereas levels of mineral N in the tithonia + fertilizer treatment were not different from the control + fertilizer treatment. At 49 DAP, maize had immobilized 85% of the applied fertilizer-n in the 0 30 cm depth (Fig. 2). The low residue quality of the maize residue favored net N immobilization. However, despite having significantly lower soil mineral N in the upper soil at 49 DAP, the maize + fertilizer treatment had equivalent rates of N uptake as compared to the other fertilized treatments. This difference cannot be explained by changes in gross mineralization rates, as maize showed a trend for lower rates than tithonia at 49 DAP. This apparent discrepancy between soil N availability and crop N uptake may be due to an extreme rainfall event immediately following the second fertilizer application. The largest rainfall event of the season (56 mm) occurred 3 d after the application of 80 kg N ha 1 and most likely leached much of the available N from the upper soil profile to lower depths. Maize root distributions measured at the same field site as our study indicate that 85% of the total root length at grain-filling occurs in the 0 60 cm depth, with only 10% being found at the cm depth and <5% beyond 90 cm (Mugendi et al., 2003). Thus, most of the N taken up by the crop should be extracted from the 0 to 60 cm depth. Furthermore, at 83 DAP, there was a trend for greater mineral N at the cm depth with the control + - fertilizer and tithonia + fertilizer treatments, indicating a leaching of N to the soil layers beyond the reach of most roots. The difference in mineral N in the cm depth between the maize + fertilizer and other fertilized treatments accounted for 76% of the difference in mineral N between these treatments noted in the 0 30 cm depth of the previous sampling time. In accordance with the direct hypothesis for interactive effects of combining fertilizer inputs (Vanlauwe et al., 2001a), combining low quality maize residue with mineral N fertilizer reduced N losses in comparison to fertilizer alone or fertilizer with high quality residue. The benefit of reduced N leaching losses from combining maize residue and fertilizer is further evidenced by the residual mineral N levels observed in the subsoil (Fig. 2). Mean season mineral N levels for the cm depth ranged from 42 kg N ha 1 for the maize treatment to 283 kg N ha 1 for the control + fertilizer treatment. This depth was also characterized by high variability with CVs for each sampling time ranging from 80% to 112%. The large variability observed supports dynamic N fluxes in this layer, but also obscures treatment effects. Therefore, fertilizer was the only significant treatment effect to increase mineral N at depth, though the difference in levels for maize + fertilizer and the other fertilized treatments averaged more than 150 kg N ha 1. These observations are supported by Vanlauwe et al. (2002), who showed reduced N leaching when fertilizer was combined with maize residue, but not when it was combined with high quality M. pruriens residue. The high N levels observed at the cm depth may be subject to further leaching into groundwater leading to potentially adverse environmental impacts, and represent a loss of N from the agroecosystem Yield interactive effects The interactive effect of combining organic residue with mineral fertilizer on crop yield and N uptake shifted from negative to positive with decreasing residue quality (Table 3). As equivalent rates of N were not applied across all inputs treatments, it is difficult to directly compare the net N uptake between the combined and individual inputs between treatments. Therefore, the interactive effect reveals whether the combining inputs decreases (negative effect) or increases (positive effect) crop yield and N uptake relative to what was expected from the sum of either input alone. The observed change in interactive effect with residue quality is consistent with the trend of mean yields from the previous 10 seasons (see Chivenge, 2008), where application rates of 1.2 C ha 1 showed negative interactive effects for tithonia and positive interactive effects for maize, though these values were not always significantly different from zero. Similarly, Mtambanengwe et al. (2006) found that interactive effects on yield were negative with high quality C. juncea, but were zero or positive with maize residue at a field site in Zimbabwe. The zero to positive interactive effect for crop yield and N uptake with the low quality residue can be explained by reduced N leaching beyond the main rooting depth when combining low quality residue with fertilizer. Additionally, a subsequent stimulation of residue-n release due to fertilizer addition (Sakala et al., 2000; Gentile et al., 2008) may have increased available N in the combined treatment as compare to the maize residue alone. However, fertilizer addition did not affect potential gross mineralization rates; although fertilizer increased gross nitrification rates at 83 DAP, indicating higher levels of available NH + 4 in these treatments. An increase in net N mineralization in the maize + fertilizer treatment may have led to increased crop N uptake, but was not observed in increased soil mineral N levels by the end of the season. Therefore, it is likely that the reduction of N leaching losses played a larger role in the positive interactive effect of combining maize residue with fertilizer than a stimulation of N mineralization. Combining high quality residue with fertilizer resulted in a small, but negative interactive effect on crop yield and N uptake. This apparent yield decrease may be because the fertilizer application stimulated residue mineralization (Gentile et al., 2008) leading to greater leaching losses, or the combined N input of high quality residue and fertilizer (213 kg N ha 1 ) exceeded crop requirements. Lower application rates of high quality residue in combination with fertilizer would further test the relationship between residue quality and crop yield interactive effects from combining inputs. However, the early season residue mineralization and lack of fertilizer-n immobilization observed with the tithonia residue in the present study indicate that residue quality alters rates of N transformation processes and will therefore likely affect interactive effects on crop yield Implications for ISFM The observed interactive benefit of combining low quality maize residue with N fertilizer by reducing system N losses has several implications for the implementation of ISFM practices. The primary benefit of combining input types in this study stemmed from the reduced loss of early season available N. This may indicate that the beneficial interactive effects of combining inputs are

7 314 R. Gentile et al. / Agriculture, Ecosystems and Environment 131 (2009) restricted to environmental and soil conditions that promote high N losses. For example, positive interactive effects of combining maize with fertilizer may only be evident under high rainfall conditions conducive to N leaching. Mtambanengwe et al. (2006) observed positive interactive effects of maize residue and fertilizer in coarse textured soils subject to N leaching. In contrast to conditions promoting positive interactive effects for low quality residues, the application of high quality residue with early season N mineralization is better suited to conditions with low risk of N leaching. Early season N release and risk of N leaching with various residue inputs has been identified as a major challenge for the coarse textured cropping systems in Zimbabwe (Chikowo et al., 2006). 5. Conclusions The availability of N during the growing season was manipulated by the use of different quality residues alone and in combination with fertilizer. High quality residue showed early season N release, which was subject to N leaching ahead of crop uptake. In contrast, low quality residue displayed immobilization of fertilizer-n and lowered early season N availability. This reduction in available N decreased leaching losses and resulted in a positive interactive effect for crop N uptake. The interactive benefit of combining low quality residue with N fertilizer indicates that this soil fertility management strategy is appropriate for environments subject to high N losses. Considering the balance of N availability and N losses, we recommend that fertilizer be combined with low quality residue under environmental conditions conducive to high leaching rates, whereas high quality residues can be applied alone under conditions with reduced risk of N leaching. 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