Post-tin-mining agricultural soil regeneration using local organic amendments improve nitrogen fixation in legume-cassava cropping system on Bangka Island, Indonesia

Post-tin mining soil in Bangka Island shows poor fertility and hence limited suitability for agricultural production. Previously, we found that organic amendments like compost, charcoal, and their combination improve the yield of intercropping cassava (Manihot esculenta Crantz) and legume species (Centrosema pubescens). We hypothesize that N-fixation by centrosema in combination with the application of organic amendments has a highly beneficial effect on crop production via improved the nitrogen content from both, the soil amendment and plant's atmospheric nitrogen fixation.

To evaluate the amount of nitrogen fixed by centrosema in post-tin mining soil; soil and crop samples were taken from different soil amendments treatments: (1) dolomite (10 t ha^-1); (2) compost (10 t ha^-1); (3) charcoal (10 t ha^-1); combined treatment of (4) charcoal+compost (10 t ha^-1 for each); and (5) charcoal+sawdust (10 t ha^-1 for each); and a control for intercropping system of local crops (Cassava and Centrosema) in July 2018. The centrosema was harvested twice during the first and second season (December 2018 and July 2019). The ¹⁵N natural abundance method was used to estimate nitrogen fixation (N₂-fixation) in centrosema at harvest time.

Comparing season and treatment, the proportion of nitrogen derived from N₂-fixation (%Ndfa) was comparable. However, the amount of N₂-fixation was significantly different due to biomass accumulation. Soil amended with compost increased N₂-fixation in centrosema by 6-fold compared to control (50 kg ha^-1), while combined treatment of charcoal and compost increased this value by 8-fold (73 kg ha^-1). When comparing the seasons, the average N₂-fixation in the first season was roughly ten times greater than in the second season (30 and 3.17 kg ha^-1, respectively). In terms of total nitrogen uptake by centrosema across seasons, soil amended with compost or charcoal+compost significantly improved total N uptake in centrosema (61 and 111 kg ha^-1, respectively). Accordingly, organic amendments, in particular in charcoal + compost treatment, significantly increased ammonium in the soil at harvest time (6.71±0.29 µg g^-1).

Our findings suggest that organic amendments, particularly combined application of charcoal and compost in post-tin mining soils can increase N₂-fixation of intercrop centrosema as well as nitrogen availability in the soil, which is of crucial importance in infertile post-mining soils.

Keywords: legume, mining, soil amendment, intercropping, nitrogen fixation