Soil nitrogen dynamics and relationships with maize yields in a gliricidia maize intercrop in Malawi

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1 Plant and Soil 211: , Kluwer Academic Publishers. Printed in the Netherlands. 155 Soil nitrogen dynamics and relationships with maize yields in a gliricidia maize intercrop in Malawi Susan T. Ikerra 1, Jumanne A. Maghembe, Paul C. Smithson and Roland J. Buresh International Centre for Research in Agroforestry (ICRAF), P.O. Box 30677, Nairobi, Kenya; 1 Present address: Mlingano Soils Research Institute, P.O. Box 50880, Tanga, Tanzania Received 7 August Accepted in revised form 18 April 1999 Key words: agroforestry, Gliricidia sepium, inorganic nitrogen, maize, nitrogen mineralization Abstract Many soils of southern Africa are severely N deficient, but inorganic fertilizers are unaffordable for most subsistence farmers. Rotations and intercrops of legumes with crops may alleviate N deficiency through biological N 2 fixation and redistribution of subsoil N to the surface. We monitored soil inorganic N dynamics for two seasons in a gliricidia [Gliricidia sepium (Jacq.) Walp.] maize (Zea mays L.) intercropin the unimodalrainfallarea of southern Malawi. One maize crop per year was grown with or without interplanted gliricidia, in factorial combination with three rates of N (0, 24 or 48 kg N ha 1 ). Application of gliricidia prunings increased (p < 0.001) topsoil (0 to 20 cm) inorganic N at the end of the dry season and during the early rains. Differences between plus and minus gliricidia treatments were less when total inorganic N to 1-m depth was summed. A greater proportion of the total inorganic N to 1-m depth occurred in the topsoil (0 to 20 cm) when gliricidia was present, suggesting that redistribution of subsoil N to the surface accounted for part of the N increase by gliricidia. Gliricidia lowered (p < 0.05) subsoil water content during drier periods. Gliricidia plots accumulated more (p < 0.01) ammonium-n during the dry season. Nitrate-N remained constant during the dry season but rose rapidly in gliricidia plots after the onset of rains. A 2-factor model including preseason inorganic N and anaerobic N mineralization potential accounted for 84% of the variability in maize yields for the two seasons data combined. The combination of preseason inorganic N and potential N mineralization appears to provide a good estimate of N supply to maize in systems receiving both organic and inorganic sources of N. Introduction Nitrogen is the most important nutrient limiting crop production in the tropics (Sanchez, 1976), but use of N fertilizers by smallholder farmers to increase crop production in sub-saharan Africa has been limited. Increases in fertilizer prices due to removal of government subsidies have further decreased fertilizer use, and population increases in many areas have decreased or eliminated use of traditional fallow systems to restore soil fertility. Southern Malawi, on the southern African plateau, is a region of high human population, where small plots of land are cropped continuously with few inputs. Yields of the staple crop maize (Zea FAX No: P.Smithson@cgiar.org mays L.) are low, averaging < 1 t ha 1 (Kumwenda et al., 1995). Consequently various low external input practices in which most of the N comes from crop rotation with grain legumes, or from agroforestry species in intercropped or rotational systems, are now being encouraged (Jones et al., 1996; Kumwenda et al., 1995). Studies from southern Africa and elsewhere confirm the importance of N nutrition in maize yields, and the potential for use of soil inorganic N and N mineralization as predictors of yields. For example, Barrios et al. (1998) obtained strong correlations in eastern Zambia between maize grain yields following 3-yr-old tree fallows and both preseason inorganic N in surface soil and soil N mineralization potential. In Uganda, Stephen (1967) reported that soil nitrate N within one

2 156 month of planting was highly correlated with maize and cotton yields, and Weber et al. (1995) in northern Nigeria obtained a highly significant relationship between maize grain yields and soil nitrate-n at 2 to 8 weeks after planting. Establishment of similar relationships in other agro-ecological zones can provide better means of predicting crop yields. In areas with a pronounced dry season, such as in southern Malawi, there is a rapid flush of inorganic N at the onset of rain (Birch, 1958). This flush is of short duration (Warren et al., 1997) due to losses from leaching (Wild, 1972), denitrification, volatilization and uptake by plants (Greenland, 1958). The magnitude of the flush depends on the length of the dry period (Birch, 1960; Semb and Robinson, 1969; Wong and Nortcliff, 1995) and the quality and quantity of organic inputs applied (Franzluebbers et al., 1995; Wong and Nortcliff, 1995). A 2-year study was initiated in 1995 in a maize gliricidia [Gliricidia sepium (Jacq.) Walp.] intercropping trial in southern Malawi (1) to monitor the seasonal dynamics of topsoil and subsoil inorganic N as affected by additions of biomass from the intercropped gliricidia and inorganic N fertilizer; (2) to examine the effect of gliricidia on soil water and on the distribution of inorganic N between topsoil and subsoil; and (3) to relate soil inorganic N and N mineralization potential to maize grain yield. Prunings from gliricidia are known to increase yields of intercrops (Amara et al., 1996). Materials and methods Site description The study was conducted at Makoka Agricultural Research Station near Zomba in southern Malawi (15 30 S, E, altitude 1030 m). The total annual rainfall ranges from 560 to 1600 mm, with a 30-year mean of 1024 mm. The rainfall is unimodal, most of it coming from December to March. The total annual rainfall for the two seasons of the study was 1221 mm in and 1602 mm in In this study, season refers to the November-to-April cropping season. The soil is classified as a Ferric Lixisol (FAO) or Oxic Haplustalf (USDA). The topsoil (0 to 20 cm) is 46% sand and 42% clay, ph (1:2.5 soil:water suspension) = 5.9, organic C = 0.88%, total N = 0.07%, bicarbonate extractable P = 26 mg kg 1, exchangeable Ca = 4.4 cmol c kg 1,Mg=1.6cmol c kg 1 and K=0.3cmol c kg 1. The effective CEC by sum of cations is 6.3 cmol c kg 1. Clay content increases with depth, but major chemical characteristics are relatively constant to > 1 m depth. At 75 to 120 cm depth, ph = 6.2, Ca = 5.1 cmol c kg 1, sand = 31% and clay = 56%. Nitrogen was the most limiting nutrient at the site. There was no response to added P at the site over six cropping seasons (Maghembe, personal communication). Experimental design and management The experiment was initiated in 1991, and the present study was started in November The experiment is a randomized complete block design with a 2 3 factorial arrangement replicated three times. The treatment factors were maize with or without intercropped gliricidia trees and N fertilizer at 0, 24 or 48 kg N ha 1. The recommended N rate for this region is 96 kg N ha 1. Plot size was 6.75 by 5.1 m, separated by 1-m wide walkways. Maize hybrid NSCM 41 was planted on 30 to 40 cm high ridges at a spacing of 0.3 m within row and 0.75 m between rows ( plants ha 1 ). Gliricidia plots consisted of 4 rows of gliricidia planted in every other furrow at a spacing of 0.9 m within row and 1.5 m between rows (7400 trees ha 1 ). In order to minimize tree root encroachment into adjacent plots or outside the experimental area, iron sheets were installed around plots to a depth of 50 cm. About 2 weeks prior to the mid-december maize planting, gliricidia trees were pruned to about 30-cm height, and the prunings were incorporated into the ridges during land preparation. (In addition, a precut occurred once or twice from July to October to remove old woody biomass and encourage new growth for later incorporation. The leaves from pre-cut material were also incorporated.) Inorganic N as calcium ammonium nitrate was split applied at 2 and 6 weeks after planting. The gliricidia trees were cut again in January about 6 weeks after planting, and the prunings were incorporated into the maize ridges during hilling operations. The mean total gliricidia biomass incorporated during the study period was 3.7 to 4.9 ton ha 1 season 1. Nitrogen content of gliricidia ranged from 2.9% to 4.9%, (average = 3.9%), corresponding to 140 to 190 kg N ha 1 season 1. Lignin content of gliricidia leaves averages about 9.9% (Adams and Gachengo, 1998). Maize was weeded twice by hand during the season. Maize was harvested from the central 5.25 by

3 m area of plots in late April of each year, and grain yield was expressed at 13% moisture content. The entire plot was harvested for gliricidia biomass estimates. Soil sampling and analysis In both years, soil was sampled in October or November before the onset of rain, at four dates during maize growth, at maize harvest in April and on two occasions during the May to November dry season. In 1997, intermittent rain occurred beginning in mid-september, totaling about 115 mm by the last sampling in November Soil samples were collected with an Edelman auger from depth increments of 0 to 20, 20 to 40, 40 to 60 and 60 to 100 cm during both seasons. In each plot, soil was collected and bulked from ten locations. Subsamples were taken to the laboratory and stored at 4 C prior to extraction within 2 days of collection. About 20 g field moist soil was extracted with 100 ml of 2 M KCl. The samples were shaken on a horizontal shaker at 250 oscillations min 1, filtered through pre-washed Whatman No. 5 filter paper and frozen until analysis. A second subsample of soil was dried at 105 C for 24 hr to determine the dry weight of extracted soil. All results are expressed on an oven-dry soil basis. Ammonium-N was determined on the extracts by a salicylate-hypochlorite colorimetric method (Anderson and Ingram, 1993). Nitrate-N was determined by cadmium reduction (Dorich and Nelson, 1984) and subsequent colorimetric analysis of nitrite. The sum of ammonium- and nitrate-n is referred to as total inorganic N in the text. Soil bulk densities were calculated from core samples (100 cm 3 ) collected at each depth from a soil pit. The bulk density was used to convert ammonium and nitrate data from mg N kg 1 to kg N ha 1. Surface soil was concentrated in the ridges during land preparation, and the 0 to 20 cm soil samples were taken solely from field ridges and not from the furrows. It is therefore likely that the estimates of N content in kg ha 1 are higher than the actual values averaged over the whole field. Any error in the estimates should be similar for all treatments, however, and treatment comparisons should therefore be unbiased. Anaerobic N mineralization potential was determined by a 7-day incubation of flooded soil at 40 C, as described by Barrios et al. (1996). Anaerobic N mineralization was calculated as the difference between the ammonium extracted from incubated and non-incubated samples, expressed in mg N kg 1 soil day 1. Aerobic N mineralization potential was determined by a 21-day incubation at 26 C of soil adjusted to 60% water-filled pore space (Barrios et al., 1996). Aerobic N mineralization was calculated as the difference in extracted ammonium + nitrate between incubated and non-incubated soils. Samples for both incubations were those collected in October 1995 and November 1996 before the rainy season. Statistical analysis A two-way analysis of variance was performed on logtransformed data of nitrate-n, ammonium-n and total inorganic N using the general linear model procedure of SAS statistical package (SAS Institute, 1990). Log transformation of the data was done to eliminate the non-homogeneity of variance, which was found with this dataset. Statistical analyses were done for each depth increment and also for the (log-transformed) sum of inorganic N for various combinations of depth increments. The resulting ANOVA tables were used to determine treatment differences for various sampling dates and depths. Statistical differences are based on analyses of log-transformed data, but means of untransformed data are presented in tables and figures. Untransformed soil water data at each date and depth were similarly analyzed to detect differences in water use among treatments. Using gliricidia dry biomass totals for different cuttings during the season, multiple regression analysis was performed relating gliricidia biomass and inorganic N rate to various measures of topsoil N and N mineralization potentials. In this analysis a stepwise regression model was used in order to remove the effect of N rates before considering biomass effects. Simple correlation coefficients were determined for the linear relationship between either soil inorganic N or N mineralization and maize grain yields. Multiple correlation coefficients were determined for the best 2-factor models relating measurements of soil N availability and grain yields. Mention of statistical significance refers to α = 0.05 unless otherwise stated. Results Seasonal dynamics of inorganic nitrogen and water Preseason topsoil (0 to 20 cm) inorganic N on 19 October 1995 before the growing season, on 8 November 1996 before the growing season

4 158 Table 1. Statistical significance for effects of gliricidia and N fertilizer on log-transformed inorganic soil N (ammonium + nitrate) in the top 20 cm at different sampling dates in southern Malawi Source of variation Gliricidia N Gliricidia N 19-Oct Dec-95 ns ns ns 18-Jan-96 ns ns ns 23-Feb-96 ns ns 15-Mar-96 ns 19-Apr-96 ns 28-Jun-96 ns ns 30-Aug-96 8-Nov-96 ns ns 10-Dec-96 ns ns 10-Jan-97 ns ns 30-Jan-97 ns ns 28-Feb-97 ns 28-Apr-97 ns ns ns 7-Aug-97 ns ns 20-Nov-97 ns ns The dry season was from April to November and the cropping season was from December to April. ns = Not significant., and refer to significance at p < 0.05, 0.01 and 0.001, respectively. Figure 1. Rainfall distribution, timing of gliricidia biomass additions (arrows), and effect of gliricidia and N fertilizer on dynamics of soil inorganic N in the topsoil and subsoil during two years in southern Malawi. and on 20 November 1997 before the growing season (Figure 1) was significantly influenced by treatments (Table 1). In October 1995 at the beginning of the growing season, mean inorganic N ranged from 26 kg N ha 1 in unfertilized sole maize plots to a maximum of 133 kg N ha 1 in fertilized gliricidia maize plots. Nitrogen fertilization alone affected (p < 0.01) preseason topsoil inorganic N for the cropping season, but accumulation of soil inorganic N was 38 to 41% lower in sole maize than gliricidia maize plots. Nitrogen fertilization had no effect on preseason inorganic N for the and cropping seasons (Table 1). Gliricidia N interactions were significant at several sampling dates (Table 1), but differences in soil inorganic N between the 24 and 48 kg N ha 1 fertilizer rates were non-significant except at a single sampling date (April 1996). Data from the 24 and 48 kg N ha 1 rates were combined for clarity of presentation in Figure 1. Gliricidia had less effect on subsoil than topsoil inorganic N (Figure 1). Analysis of variance using the sum of inorganic N at 40 to 100 cm depth showed significant differences due to gliricidia for only three out of 13 dates with sufficient sampling depth. By comparison, gliricidia significantly affected topsoil inorganic N on ten of the dates. Total inorganic N to 1-m depth was also less affected by gliricidia treatments, with four dates out of 13 showing significant differences due to gliricidia. The fraction of the total inorganic N to 1-m depth that occurred in the topsoil (0 to 20 cm) was compared for gliricidia and sole maize plots, using only the zero N rate (Figure 2). The topsoil fraction of the total was consistently higher in gliricidia plots, and was significantly different on four dates (Figure 2). Soil water content to 1-m depth was significantly lower in gliricidia plots on several sampling dates. The

5 159 Figure 2. Fraction of the total inorganic N in soil to 1-m depth occurring in the 0 to 20 cm layer, as affected by gliricidia intercropping. Only data from the treatments without added inorganic N fertilizer are presented., refer to significant difference in topsoil fraction at p <0.05or 0.01, respectively. most consistent differences were in the subsurface (20 to 100 cm) layers during the latter part of the rains and during the dry season (March through November). Significance levels ranged from p <0.01to0.05on7 of 13 dates. On the dates when there were significant decreases, gravimetric water content ranged from 2.2 to 6.0% lower in gliricidia plots (average = 3.3%), a 12 to 44% decrease from sole maize plots (average = 20%). The significant treatment differences in topsoil inorganic N near the end of the dry season in October 1995 (Table 1 and Figure 1) decreased rapidly upon the onset of rains in mid-november and became nonsignificant by mid-december Treatment differences continued to be non-significant up to February During the April to November 1996 dry season, topsoil inorganic N remained higher in the gliricidia plots (Table 1). Ammonium increased (p < 0.001) during the dry season, but topsoil nitrate did not. At the beginning of the rainy season in December 1996, there was a close association between inorganic N flush and incidence of rainfall. The first significant rainfall of 21 mm was received on 5 December 1996 after which there was a significant (p < 0.001) increase in topsoil nitrate, which was detected five days later on 10 December Topsoil nitrate N increased from 5.9 to 21 kg N ha 1 in the sole maize plots and from 7.7 to 69 kg N ha 1 in gliricida maize plots during this period. As in the previous season, the increase occurred within a short period and then decreased rapidly. Topsoil nitrate N decreased to less than 10 kg N ha 1 in all treatments by the end of January 1997, 5 weeks after planting. The decrease in topsoil inorganic N was accompanied by a corresponding increase in subsoil N, which peaked in mid-january 1997 (Figure 1). By maize harvest in late April, the nitrate content of the topsoil had decreased to between 3 and 6 kg N ha 1. During the 1997 dry season there was again a gradual increase in inorganic N, primarily as ammonium N. The accumulation of topsoil N during the dry season (April and August samplings) was again significant (p < 0.001) for ammonium and nonsignificant for nitrate. Gliricidia plots had the highest ammonium accumulations. Fertilization alone did not significantly affect inorganic N accumulations during the dry period (Table 1). The final sampling in November 1997 followed about 115 mm of unusually early rains, and a substantial inorganic N flush was again apparent. Both ammonium and nitrate increased between August and November 1997, but only the nitrate increase was significant (p < 0.001). The increase was greatest for gliricidia plots, and was not significantly different among N rates. Nitrogen mineralization Both aerobic and anaerobic N mineralization in topsoil were significantly affected by gliricidia in 1996 but not

6 160 Table 2. Effect of prunings from intercropped gliricidia trees and N fertilizer on topsoil (0 to 20 cm) N mineralization parameters before the start of the December-to-April cropping season in southern Malawi Treatment Anaerobic N Aerobic N mineralization (mg mineralization (mg Nkg 1 day 1 ) Nkg 1 day 1 ) Sole maize, no N Sole maize, with N Gliricidia maize, no N Gliricidia maize, with N Source of variation Significance level Gliricidia 0.34 < <0.001 N Gliricidia N in Prunings from intercropped gliricidia trees significantly (p < 0.001) increased aerobic and anaerobic mineralization in Gliricidia plus inorganic N fertilization gave no increases in N mineralization (Table 2). Considering only sole maize plots, inorganic N fertilization tended to increase N mineralization, but the differences were non-significant except for aerobic mineralization in 1996 (p = 0.03). Correlation of soil N parameters with gliricidia biomass and inorganic N additions gave several significant relationships, with the most consistent relationships being with anaerobic N mineralization potential. The best correlation was between 1996 anaerobic N mineralization and total gliricidia biomass added prior to planting, in a stepwise model that included N fertilizer rate, gliricidia biomass and N fertilizer rate biomass (p < 0.01, R 2 = 0.879, N = 9): N mineralization in mg kg 1 day 1 = (N fertilizer rate in kg ha 1 )+ 0.55(preseason biomass in t ha 1 ) (N fertilizer rate biomass) Relationship between soil and plant N and maize grain yield Preseason topsoil ammonium, nitrate and total inorganic N correlated with maize grain yields in both the and seasons (Table 3), but correlations were much higher in (r 2 = 0.92) than in (r 2 = 0.34). The slopes of the yield vs. inorganic N lines were also quite different between seasons (Figure 3). Combining data for the two seasons gave a linear relationship (r 2 = 0.72) between preseason inorganic N and maize grain yield (Figure 3). Including preseason subsoil N by summing to different depth increments generally resulted in poorer correlations than with topsoil N alone. Both aerobic and anaerobic N mineralization potential of preseason samples were significantly correlated with maize yield in , but not in Maize grain yield was better related to anaerobic than to aerobic N mineralization in (Table 3). Maize grain yield appeared to be a function of both preseason inorganic N and mineralization potential. A multiple regression using preseason inorganic N and anaerobic mineralizable N gave a better predictability of yield (R 2 = 0.84) than inorganic N alone, when data from both seasons were combined (Figure 4). Discussion Seasonal inorganic N and water dynamics The significant effect of prunings from intercropped gliricidia on preseason inorganic N that was observed just before the start of the season (October 1995) and of the season (November 1996) presumably resulted from mineralization of gliricidia biomass (Mwiinga et al., 1994; Palm and Sanchez, 1991). The difference between the amounts of preseason inorganic N between the two seasons may be

7 161 Table 3. Ranges in mean measures of preseason soil N and their correlation with maize grain yield for and seasons in southern Malawi Parameter season season Minimum Maximum r 2 Minimum Maximum r 2 Soil N (kg N ha 1 ) Ammonium Nitrate Inorganic N N Mineralization (mg N kg 1 day 1 ) Anaerobic ns Aerobic ns ns = Not significant., and designate significance at p< 0.05, 0.01 and 0.001, respectively. related to the previous season s rainfall and crop yield. The season was preceded by a severe drought period that occurred in (650 mm total rainfall from September 1994 through August 1995), with little leaching and near-zero maize yield (ICRAF, 1998); this resulted in the substantial accumulation of preseason inorganic N observed in October The total rainfall from December 1995 to April 1996 (1221 mm) was above average, hence more of the inorganic N could have been lost through leaching and denitrification, as well as greater crop offtake, thus leading to the lower preseason inorganic N that was observed in November The low mineralization potentials measured for the season may be partly explained by the high 1995 preseason inorganic N levels, in that mineralization is measured by difference between post- and pre-incubation samples. The high pre-incubation N levels caused a large background that could obscure the relatively small increases due to mineralization. Wetting of the soil in December 1996 after the dry season resulted in an increase in available N as earlier reported by several researchers (Semb and Robinson, 1991; Warren et al., 1997; Wong and Nortcliff, 1995). This is attributed to an increase in net N mineralization upon moistening of dry soils (Birch, 1960). The magnitude of the flush depended on the organic matter content and length of the preceding dry period. The N flush was higher in gliricidia plots where there was more (p < 0.001) total C and N in the 0 to 20 cm layer (mean over both seasons = 0.88% C, 0.07% N) due to application of gliricidia biomass, than in sole maize plots (mean = 0.65% C, 0.05% N). The N flush potential of the soil can be approximated by laboratory mineralization potentials, which were higher in gliricidia plots (Table 2). The significant correlation of anaerobic mineralization potential with added gliricidia biomass also gives an indication of the influence of biomass additions on potential N supply. Similar effects have been reported by Wild (1972), who measured the largest flush in plots that had received farmyard manure in Nigeria. A similar flush presumably occurred at the start of the season, but was not detected, perhaps due to the length of time between samplings. Alternatively, the very large accumulation of inorganic N in October 1995 may have obscured any N flush resulting from mineralization of organic matter. The accumulation of ammonium N that was observed during the dry period perhaps indicates addition from lysis of microbial cells, decomposition of maize roots and mineralization of soil organic matter, combined with lack of nitrification. Near-constant nitrate N concentration observed throughout the dry period further implies that nitrification is more sensitive than ammonification to soil water deficit. Ammonification is less limited by water deficit because actinomycetes and fungi responsible for this process are able to remain active at lower water potentials, unlike nitrifying bacteria that are more limited by water deficit (Wetselaar, 1968). Similar results were obtained by Robinson (1957) who reported accumulations of 55 kg ammonium-n ha 1 in the 0 to 60 cm soil layer in Kikuyu red loams of Kenya at 1.5 MPa, but nitrification stopped completely at a matric potential greater than 1.5 MPa. The rapid decline in topsoil inorganic N that occurred early in the rainy season during both

8 162 Figure 3. Relationship between preseason soil inorganic N in the top 20 cm and yield of the subsequent maize crop for two seasons in southern Malawi. years might be mostly due to movement down the soil profile (Figure 1), denitrification and microbial immobilization rather than to plant uptake since the crop demand for N at this time is low (Sanginga et al., 1995). Although we did not estimate the contribution of N 2 -fixation by gliricidia to the higher measured topsoil N, subsoil N measurements and the fraction of the 1-m total in topsoil (Figures 1 and 2) indicate that part of the higher topsoil N is due to recycling of leached N from deeper layers. Other evidence for a recycling component is the significantly higher topsoil exchangeable K in gliricidia plots (p < in 1995 and p < 0.05 in 1996; data not shown). Hartemink et al. (1996) found that 18-month tree fallows depleted subsoil N in western Kenya, which after deposition on the surface became available for succeeding maize crops. The relative amount of N added from N 2 -fixation vs. N recycled from subsoil layers in these systems, will be important in determining the longer-term sustainability of such systems. The higher water use in gliricidia plots compared to sole maize plots serves to point out that the added N from gliricidia biomass comes at a cost. Potential N supply is related to biomass accumulation, which in turn requires sufficient soil water to support growth. The lower limits of rainfall and soil water storage that can support sufficient growth during the dry season are not yet known. Relationship between N parameters and maize grain yield Preseason inorganic N alone predicted maize grain yield well in the first season, which was preceded by a drought period and was accompanied by a large accumulation of inorganic N. In the more normal season, grain yield was better related to measures of potentially available N such as anaerobic and aerobic N mineralization. When the seasonal variability of biomass inputs, rainfall and crop offtake are considered jointly, maize grain yield seems to be better explained by a combination of both preseason inorganic N and the mineralization potential, as revealed by the strong multiple correlation coefficient between preseason inorganic N and anaerobic N mineralization potential with maize grain yield. This observation is in conformity with that of Barrios et al. (1998). Considered alone, preseason anaerobic mineralization was better correlated with maize yield than preseason aerobic mineralization. This is in agreement with the findings of Maroko et al. (1998), who examined four contrasting pre-maize treatments in Kenya. Barrios et al. (1998), however, obtained a better correlation between maize grain yield and preseason aerobic rather than anaerobic N mineralization when maize followed treatments that included vegetation with high C-to-N ratio. In their case, residues with high C-to-N ratio apparently resulted in net N

9 163 Figure 4. Maize yield for two seasons in southern Malawi estimated by a 2-factor model including preseason inorganic N (kg N ha 1 )and anaerobic N mineralization potential (mg N kg 1 day 1 ). mineralization during anaerobic incubation but in net N immobilization under field conditions. Conclusion Intercropping maize with gliricidia and addition of gliricidia prunings to the maize significantly increased topsoil preseason inorganic N. The effect was more pronounced in a year preceded by long drought. Differences among gliricidia and sole maize treatments were less when total inorganic N to 1-m depth were considered, indicating that part of the enhanced topsoil inorganic N with gliricidia was due to recycling from depth. Gliricidia maize intercropping depleted stored soil water more than sole maize during the dry season. Growth of gliricidia with addition of prunings resulted in significant accumulations of ammonium N during the dry season, indicating that ammonification was less limited than nitrification by low soil water. There was a strong correlation between maize grain yield and both preseason inorganic N and anaerobic N mineralization potential. A combination of preseason inorganic N and some measure of potential N supply is likely to be useful in predicting yield performance of systems with integrated use of organic and inorganic N sources. Acknowledgments The work reported here was funded by the Canadian International Development Agency (CIDA). Financial support for the first author s participation in this study was provided by the Ford Foundation. Statistical support was provided by Mr R Coe. Staff of Makoka Research Station in Malawi provided assistance, especially Mr Bentry Simwaka in the field and Mr Majidu Mdala in the laboratory. References Adams E S and Gachengo C 1997 Organic resource database. Tropical Soil Biology and Fertility Programme, Nairobi, Kenya. Anderson J M and Ingram J S I 1993 Tropical soil biology and fertility. A handbook of methods. Second edition. CAB International, Wallingford, Oxon, UK. 221 p. Amara S D, Sanginga N S, Danso KA and Suale D S 1996 Nitrogen contribution by multipurpose trees to rice and cowpea in alley cropping system in Sierra Leone. Agrofor Syst 34, Barrios E, Buresh R J and Sprent J I 1996 Nitrogen mineralization in density fractions of soil organic matter from maize and legume cropping systems. Soil Biol. Biochem. 28, Barrios E F, Kwesiga F, Buresh R J, Sprent J I and Coe R 1998 Relating preseason soil nitrogen to maize yield in tree legume maize rotations. Soil Sci. Soc. Am. J. 62, Birch H F 1958 The effect of soil drying on humus decomposition and nitrogen availability. Plant Soil 10, Birch H F 1960 Nitrification in soils after different periods of dryness. Plant Soil 12, Dorich R A and Nelson D W 1984 Evaluation of manual cadmium reduction methods for the determination of nitrate in potassium chloride extracts of soil. Soil Sci. Soc. Am. J. 48,

10 164 Franzluebbers A J, Hons F M and Zuberer D A 1995 Tillage and crop effects on seasonal soil carbon and nitrogen dynamics. Soil Sci. Soc. Am. J. 59, Greenland D J 1958 Nitrate fluctuations in tropical soils. J. Agric. Sci. 50, Hartemink A E, Buresh R J, Jama B and Janssen B H 1996 Soil nitrate and water dynamics in sesbania fallows, weed fallows, and maize. Soil Sci. Soc. Am. J. 60, Jones R B, Snapp S S and Phombeya S K 1996 Management of leguminous leaf residues to improve nutrient use efficiency in the sub humid tropics. In Driven by Nature. Plant Litter Quality and Decomposition. Eds G Cadisch and K E Giller. pp CAB International, Wallingford, UK. Kumwenda J D T, Waddington S R, Snapp S S, Jones R B and Blackie M J 1995 Soil fertility management in the smallholder maize based cropping systems of Africa. In The Emerging Maize Revolution in Africa: The Role of Technology, Institutions and Policy. Michigan State University, USA, 9 12 July, ICRAF 1998 ICRAF Annual Report for International Centre for Research in Agroforestry, Nairobi, Kenya. Maroko J B, Buresh R J and Smithson P C Soil nitrogen availability as affected by fallow maize systems on two soils in Kenya. Biol. Fertil. Soils 26, Mwiinga R D, Kwesiga F R and Kamara C S 1994 Decomposition of leaves of six multipurpose tree species in Chipata, Zambia. For. Ecol. Manage. 64, Palm C A and Sanchez P A 1991 Nitrogen release from the leaves of some tropical legumes as affected by their lignin and polyphenolic contents. Soil Biol. Biochem. 23, Robinson J B D 1957 The critical relationship between soil moisture content in the region of wilting point and mineralization of natural soil nitrogen. J. Agric. Sci. 49, Sanchez P A 1976 Properties and management of soils in the tropics. John Wiley and Sons, New York. Sanginga N B, Vanlauwe B and Danso S K A 1995 The management of biological N fixation in alley cropping systems, estimation and contribution to nitrogen balance. Plant Soil 174, SAS Institute SAS/STAT User s Guide, Vol. 2. Version 6.1. SAS Institute, Cary, NC, USA. Semb G and Robinson J B D 1969 The natural nitrogen flush in different arable soils and climates in East Africa. E. African Agric. For. J. 34, Stephen D 1967 Effects of grass fallow treatments in restoring soil fertility of Buganda clay loam in South Uganda. J. Agric. Sci. (Cambridge) 63, Warren G P, Atwal S S and Irungu J W 1997 Soil nitrate variations under grass, sorghum and bare fallow in semi-arid Kenya. Exp. Agric. 33, Weber G, Chude V, Pleysier J and Oiketh S 1995 On farm evaluation of nitrate nitrogen dynamics under maize in the northern guinea savanna of Nigeria. Expl. Agric. 31, Wetselaar R 1968 Soil organic nitrogen mineralization as affected by low soil water potential. Plant Soil 29, Wild A 1972 Nitrate leaching under bare fallow at a site in northern Nigeria. J. Soil Sci. 23, Wong M T F and Nortcliff S 1995 Seasonal fluctuations of native available N and soil management implications. Fert. Res. 42, Section editor: R Merckx