Soil nitrogen status and turnover in subtropical leucaena-grass pastures as quantified by δ15N natural abundance

Grass pasture rundown results from a depleted store of soil mineral nitrogen (N) over time. In these systems, mineral N is retained in the standing plant biomass, and /or immobilized in the soil organic matter. Leucaena leucocephala spp. glabrata is a leguminous shrub which is incorporated into grazing systems to provide a high protein fodder for beef cattle production. Leucaena can also enhance the soil N status through biological N2 fixation and easily mineralisation of labile leucaena residues. However, there is limited information on the soil N dynamics of leucaena-grass system especially in the subsoil layers of the soil profile, including the primary N source for leucaena, and the effect of leucaena residue on accompanying green panic (Panicum maximum var. trichoglume). We quantified the effect of pasture-type, soil-type and leucaena stand age on soil N stocks and δ15N values at different depths. A chronosequence of leucaena stands (0-40 years) and paired grass sites were sampled to 1.0 m depth. Using the δ15N natural abundance technique, it was estimated that 73% of leucaena N originated from atmospheric N2-fixation. Total soil N stocks were 45% higher beneath leucaena rows compared to the paired grass sites at 0-0.1 m depth. In addition, total soil N stocks increased by 32% in the surface at 0-0.3 m depth, attaining a maximum in the 40-year old stand. The N dynamics beneath the leucaena rows and mid-rows suggest a tight cycling of available N within the system. This study further highlights the importance of incorporating legumes into grazing systems to increase plant available N and mitigate pasture rundown.