Improving nitrogen use efficiency by maize through the pigeon pea-groundnut intercrop-maize rotation cropping system in Malawi

A study was initiated in the 2011/12 cropping season with a parallel experiment mounted along side in the second season to investigate the possibility of improving nitrogen use efficiency (NUE) by the maize crop in a pigeon pea groundnut intercrop-maize rotation cropping system at Chitedze Agricultural Research Station in Malawi. The parallel experiment was conducted to compare the performance of legumes over two cropping seasons. The experiments involved the planting of two pigeon pea varieties, namely; long (ICEAP 04000) and medium duration (ICEAP 00557) and groundnut (CG 7) as monocultures or as intercrops. The main experiment had eight treatments; 1) Sole maize (control); 2) Medium duration pigeon pea; 3) Long duration pigeon pea; 4) Sole groundnut; 5) Medium duration pigeon pea + groundnut; 6) Long duration pigeon pea + groundnut; 7) Medium duration pigeon pea + groundnut (biomass not incorporated in season two); and 8) Long duration pigeon pea + groundnut (biomass not incorporated in season two). All the treatment plots except treatment plot number one were treated with 25 kg P ha -1 . The parallel experiment had ten treatments; 1) Long duration pigeon pea; 2) Medium duration pigeon pea; 3) Sole groundnut; 4) Sole groundnut + 25 kg P ha -1 ; 5) Medium duration pigeon pea 25 kg P ha -1 ; 6) Long duration pigeon pea + 25 kg P ha -1 ; 7) Long duration pigeon pea + groundnut; 8) Long duration pigeon pea + groundnut + 25 kg P ha -1 ; 9) Medium duration pigeon pea + groundnut; and 10) Medium duration pigeon pea + groundnut + 25 kg P ha -1 . Both experiments were laid in a randomized complete block design replicated three times. Key parametres assessed during the experiment included; legume biomass and grain yield, soil nitrate nitrogen ( NO 3- ‒N), maize stover, and rachids yields; nitrogen and phosphorus partitioning both for the legumes and maize NUE.ii Soil characterization was conducted before treatment application in the first and second year. Generally, the soil chemical characteristics for soil samples collected in all the treatment plots both in the main and parallel experiment indicated that the soil was of low fertility. The %OC and total N (%) was low, and was at 1.4 %, 0.12%, respectively, while plant available phosphorus (Mehlich 3) was marginally adequate (19 mg P kg -1 to 25 mg P kg -1 ). The soil texture which was predominantly sandy clay loam to sandy clay suggest potential high leacheability of mobile nutrient ions more especially nitrogen as nitrate. Inevitably, if the soil is not properly managed crop yield could be reduced drastically. Total biomass yield assessment for the pigeon pea was conducted in the parallel experiment in season two. Partial biomass yield assessment was done in season one in the main experiment. In season two this involved assessment of litter, twigs, stems, fresh leaves and roots for each treatment plot. The litter was collected from the ground on one planting station (90 cm x 75 cm). This was done in September, 2013. Fresh leaves, twigs and stems were also weighed from the 2 m x 2 m net plot. These were oven dried for 72 hours at 70 o C to constant weights. The assessment of the above ground groundnut biomass indicate a low yield range of 479-656 kg ha -1 while the assessment of the total above ground biomass yield of the pigeon pea varieties indicate a high yield range of 3,124-3,840 kg ha -1 . Nitrogen yield assessment indicate that the monoculture for groundnut treated with P yielded more N (52.0 kg N ha -1 ) compared to the non treated groundnut monoculture (40.0 kg N ha -1 ) while the P treated monoculture for the long duration pigeon pea yielded higher soil returnable N (87.2 kg N ha -1 ) compared to the non P treated counterpart (79.7 kg N ha -1 ). For the medium duration pigeon pea monoculture higher soil returnable N was harvested in the P treated monoculture (95.6 kg N ha -1 ) thaniii the non P treated monoculture (87.0 kg N ha -1 ). Similar soil returnable yield of N was observed in the P (128.3 kg N ha -1 ) and non P treated (128.8 kg N ha -1 ) intercrop of medium duration pigeon pea and groundnut. Higher soil returnable yield of N was observed in the P (128.4 kg N ha -1 ) and non P treated (103.9 kg N ha -1 ) intercrop of long duration pigeon pea and groundnut. Generally, the monocultures and intercrops treated with P gave higher N yield when compared to the non P treated counterparts. This was attributed to enhanced biological N fixation in the P treated treatments due to the increased level of available P. Poor grain filling for the pigeon pea varieties was observed both in the main and parallel experiment. For the groundnut shells’ yield ranged from 846 kg ha -1 to 1,985 kg ha -1 while grain yield ranged from 1,513 kg ha -1 to 3,025 kg ha -1 and haulms’ yield ranged from 1,396 kg ha -1 to 2,463 kg ha -1 . N concentration in the shells ranged from 0.9% to 1.5% while in the grain ranged from 2.9% to 3.2% while for haulms ranged from 1.9% to 2.3%. N yield in the groundnut shells ranged from 10.2 kg N ha -1 to 25.2 kg N ha -1 while for grain ranged from 46.9 kg N ha -1 to 98.8 kg N ha -1 and for haulms ranged from 29 kg N ha -1 to 52 kg N ha -1 . The concentration of N in the maize grain ranged from 1.1% to 2.1% while maize grain yield ranged from 1,775 kg ha -1 to 5, 806 kg ha -1 and the N yield ranged from 23 kg N ha -1 to 115 kg N ha -1 . The concentration of N in the maize stover ranged from 0.1% to 1.0% while stover yield ranged from 2,029 kg ha -1 to 4,413 kg ha -1 and the N yield ranged from 2.3 kg N ha -1 to 33.2 kg N ha -1 . The concentration of N in the maize rachids ranged from 0.1% to 0.5% while the rachids yield ranged from 405 kg ha -1 to 1,235 kg ha -1 and the N yield ranged from 0.7 kg ha -1 to 5.1 kg N ha -1 . The data indicated that more N in the groundnut and maize plant is translocated to the grain as such there is net export of N from the field which might lead to depletion of N in the soils.iv Assessment of soil NO 3- ‒N was conducted in the main experiment in the 2012/2013 cropping season, after the emergence of the succeeding maize crop. This was done in order to establish the effect of incorporating legume residues on soil NO 3- ‒N and the implication this might have on nitrogen management and crop yield. Data was collected over a period of three weeks. This was done before top dressing with urea. Over the study period high levels (100 > mg L -1 ) of soil NO 3- ‒N were observed that were in most cases statistically the same (p>0.05) across the treatment plots. In general, mean soil NO 3- ‒N was higher between 20 cm to 40 cm than 0 to 20 cm, attributable to the soil texture which is predominantly sandy clay loam both between 0 to 20 cm and 20 cm to 40 cm hence high leaching of NO 3- . Most likely, the level of soil NO 3- ‒N, into the season, in treatment plots in which no biomass was incorporated declined faster than in treatment plots where no incorporation was done, as a result of uptake by the maize crop and leaching losses. The high levels of soil NO 3- ‒N probably, lasted longer into the season for the latter treatment plots, but might not have endured until the end of the cropping cycle due to limited supply of N from the incorporated biomass. Therefore, supplementation of N from mineral sources is requisite for the attainment of optimal maize grain yield. In general the KCl method gave higher readings of NO 3- ‒N (0-20 cm=90.3 mg L -1 and 20 cm to 40 cm =108.5 mg L -1 ) compared to the nitrate meter (0 to 20 cm=68.1 mg L -1 and 20 cm to 40 cm=65.9 mg L -1 ). This could be attributed to the differences in the extraction procedure for the two methods, a cause of the different results generated by each procedure. Assessment of NUE for the maize crop was conducted in order to determine how efficient the crop utilized applied N from urea. NUE was determined using the recoveryv efficiency (RE), agronomic efficiency (AE) and partial factor productivity (PFP) indices. Under the conditions of this study RE ranged between 20% and 88%, AE ranged between 7 and 32 kg yield increase per kg of nitrogen applied and PFP ranged from 27 to 104 kg grain yield per kg nutrient applied. RE values of 50% to 80% , AE values of 10–30 kg kg - 1 and PFP values of 40–80 kg kg -1 are often encountered with values >25 kg kg -1 for AE and >60 kg kg -1 for PFP being common in well-managed systems or at low levels of N use, or at low soil N supply. The linear increase in grain yield with application of N and the presence of a diminishing-return relationship between maize grain yields (grain yield was near the yield potential of the maize variety at high N input) and increasing nitrogen supply, suggest that the RE, AE and PFP values emerging from this study might apply both to low and high levels of N use, or at low and high soil N supply. From the study, the following conclusions were made; the soils on which the experiments were conducted were of low fertility status evidenced by the low nitrogen and phosphorus. A situation that calls for soil N and P management for increased crop productivity. Furthermore, the study confirmed the viability of the pigeon pea-groundnut intercropping system. The nitrogen yields for the cropping system were deemed to be reasonably high. Employing this system in rotation with maize can reduce to an extent the amounts and hence the costs of mineral fertilizers required for maize production. On the effect of incorporating legume biomass into the soil on soil NO 3- ‒N, it was noted that apparently the soil had high NO 3- ‒N in the soil solution attributable to residual N from N- fertilization and legume cropping over years. Soil NO 3- ‒N was higher between 20 cm to 40 cm than between 0 to 20 cm in the soil. This was attributable to the soil texture which is predominantly sandy clay loam with low to medium level of SOM. Leaching of NO 3- isvi high under such soil conditions. It is likely that soil NO 3- ‒N levels in all the treatment plots would decline in all the treatment plots along the season principally due to crop uptake of N and leaching losses. This for the Malawian smallholder farmers implies that in this cropping system N supplementation from mineral fertilizer is not optional if reasonably high maize yield is to be realized. Additionaly, comparative analysis of two soil NO 3- ‒N analysis procedures indicated that the KCl method gave higher readings of NO 3- ‒N compared to the nitrate meter. This accrued from the differences in the extraction procedure for the two methods. The study served to confirm that more N yield in groundnut is exported from the field in form of shells and grain and less is returned to the soil upon incorporation of the haulms. Over and above, it was observed that in the pigeon pea much of the N contribution to the soil N pool comes from the above ground biomass as compared to the below ground biomass. Additionally, supply of P to legumes increases N accumulation and yield through enhanced biological N fixation. The legumes, however, do not yield enough P for the correction of soil P deffiencies that are prevalent across Malawi. The PFP ( 27 to 104 kg grain yield kg -1 N applied ) values obtained under the conditions of this study, which are higher than that ( 20 kg grain yield kg -1 N applied ) reported under smallholder farms in Malawi, seem to suggest that legume biomass incorporation into the soil does improve NUE of the suceeding maize crop. The NUE values generated fall within the range of values that are often encountered in well-managed systems or at low levels of N use, or at low soil N supply. Ratooning of the pigeon pea in this environment appears to be the solution to the observed poor grain filling for the pigeon pea. Furthermore, the low P yields from thevii legumes indicate the need to supply P using mineral fertilizer sources in addition to N for optimal maize grain yield. Further studies in this cropping system should focus on understanding the decomposition and mineralization pattern of the incorporated legume biomass for the assertion of the time and amount of N release. This is critical inorder to establish if this is in syncrony with nutrient demand by the maize crop.