Synchronization of nitrogen mineralization with N update through maize stover placements and N fertilization under continous maize-monocropping systems in Kenya

One of the factors limiting maize (Zea mays L.) yields in continuous maize monocropping systems, is the temporal and spatial mismatch between the availability of plant nutrients, notably nitrogen in the soil solution, and the maize growth N uptake demands (ASYNCHRONY). In this study, the potential to match soil N mineralization with N uptake requirements of maize (as a test crop) , for optimal growth and development (SYNCHRONY), was investigated in a randomized complete block design field experiment (replicated four times) consisting of fi ve stover (4 tha-1) placement regimes (removal, surface mulch, mixed, shallow and deep incorporation), each in the absence and presence of N fertilizer (50 kg ha-1), in a semi-humid and semi­ arid agro-ecological zones in Kenya, over six successive seasons (1991 short rains to 1994 long rains). In order to test synchrony, soil biological processes of decomposition and mineralization were studied under the framework of the Tropical soil Biology and Fertility (TSBF) Programme which uses standard methodology. In this approach synchrony was determined as a measure of the magnitude of nitrogen use efficiency. Mass loss and N loss was fastest and slowest with stover incorporation and surface mulch, respectively (other treatments being intermediate between the two extremes). N fertilization increased rate of mass loss and N release, although not with a magnitude likely to effect synchrony through litter J decomposition per se, but rather stimulate~ microbial activity Jl I and advancerJonset of net mineralization. Termites contributed to surface mass loss by a factor of between 1.5 and 3.1 at NDFRC and NARL, respectively. My results show that it is not possible to achieve synchrony of N mineralization with crop N uptake in the first season following application of 4 tha-1 of maize stover (C:N ratio 30) at the two sites. The magnitude of initial synchrony however depends on the resource and soil C:N ratio, soil clay content and diversity and abundance of the comminutors. At NARL (67.2% clay), surface mulching initially resulted in synchrony and later in the season, asynchrony. This is in contrast with incorporation which result d in asynchrony throughout the season. Thereafter, at NARL (and all seasons at NDFRC) synchronous mineralization of N with maize N demand was observed under stover incorporation treatment,but surface mulch (at both sites) resulted in either initial synchrony and late season asynchrony, or initial asynchrony and late synchrony, probably resulting from the incorporation of the undecomposed mulch during tillage, at the beginning of the following season. In the context of ni"trogen use efficiency, treatments exhibi ting synchrony ( eg stover incorporation, season 2 at NARL, and all seasons at NDFRC) resulted in higher nitrogen use efficiency (in terms of agronomic efficiency and nitrogen recovery efficiency), than treatments exhibiting ~synchrony. On long-term basis, increasing percent of incorporated stover, eg, ( "by maize, \{which resulted in higher productivity and higher nitrogen use efficiency, than in treatments exhibiting Asynchrony (eg surface mulch and mixed placement). Further research needs to be undertaken to overcome the latter scenario of c. Isynchrony and low nitrogen use efficiency.