Maize productivity and mineral N dynamics following different soil fertility management practices on a depleted sandy soil in Zimbabwe

There is a need for an improved understanding of nitrogen (N) dynamics in depleted sandy soils in southern Africa. A field experiment was conducted to evaluate the performance of different soil fertility improvement practices on a degraded granitic sandy soil in Zimbabwe. Legumes capable of accumulating large amounts of N through biological N2 fixation and subsoil N capture were tested against soybean/maize rotation, cattle manure fertilization and continuous maize (Zea mays L) with or without fertilizer. Soybean (Glycine max) accumulated 82 kg ha-1 N (seed+stover), while mucuna (Mucuna pruriens) produced 87 kg ha-1 N in its biomass. Soybean fixed 76% of its N, while mucuna fixed 96% of the accumulated N as estimated by the 15N natural abundance method. Although the following maize crop in the second season suffered from drought stress, maize N uptake was 14.8 kg ha-1 following soybean and 16.4 kg ha-1 following mucuna, compared with 5.2 kg ha-1 for the unfertilized maize and 25.6 kg ha-1 for the maize fertilized with N at 90 kg ha-1. Cajanus cajan and Crotalaria paulina added barely 10 kg ha-1 of N through their biomass and had no effect on N uptake by maize. Apparent recovery of the added N by maize was 47% for the fertilized maize, 36% for soybean, 12% for mucuna and 9% for cattle manure. There was very little partitioning of N into grain and uptake was mostly before the onset of the drought. Despite the large differences in added residue N, differences in soil mineral N were only evident up to 4 weeks after the beginning of the rains, after which mineral N concentrations became very small in all treatments due to leaching, rather than crop uptake. By the eighth week after crop emergence, maize root length density had increased to about 0.1 cm cm-3 at the 60–80 cm depth, the rapid increase apparently stimulated by the drought. It was concluded that mineral N available to maize from the residues tested falls short of what is required to sustain high maize yields. In these environments where biomass accumulation in many legumes is restricted by soil biophysical factors (poor nutrient concentrations, acidity, coarse texture), combinations of legume rotations and mineral N fertilization will remain the most viable option for sustainable agriculture