NITROGEN MINERALIZATION AND AVAILABILITY OF MIXED LEGUMINOUS AND NON-LEGUMINOUS COVER CROP RESIDUES IN SOIL

Whereas non-leguminous cover crops such as cereal rye (Secale cereale) or annual ryegrass (Lolium multiflorium) are capable of reducing nitr ogen (N) leaching during wet seasons, leguminous cover crops such as h airy vetch (Vicia villosa) improve soil N fertility for succeeding cro ps. With mixtures of grasses and legumes as cover crop, the goal of re ducing N leaching while increasing soil N availability for crop produc tion could be attainable. This study examined net N mineralization of soil treated with hairy vetch residues mixed with either cereal rye or annual ryegrass and the effect of these mixtures on growth and N upta ke by cereal rye. Both cereal rye and annual ryegrass contained low to tal N, but high water-soluble carbon and carbohydrate, compared with h airy vetch. Decreasing the proportion of hairy vetch in the mixed resi dues decreased net N mineralization, rye plant growth and N uptake, bu t increased the crossover time (the time when the amount of net N mine ralized in the residue-amended soil equalled that of the non-amended c ontrol) required for net N mineralization to occur. When the hairy vet ch content was decreased to 40% or lower, net N immobilization in the first week of incubation increased markedly. Residue N was significant ly correlated with rye biomass (r=0.81, P<0.01) and N uptake (r=0.83, P<0.001), although the correlation was much higher between residue N a nd the potential initial N mineralization rate for rye biomass (r=0.93 , P<0.001) and N uptake (r=0.99, P<0.001). Judging from the effects of the mixed residues on rye N Concentration and N uptake, the proportio n of rye or annual ryegrass when mixed with residues of hairy vetch sh ould not exceed 60% if the residues are to increase N availability. Fu rther study is needed to examine the influence of various mixtures of hairy vetch and rye or annual ryegrass on N leaching in soil.