Soil organic matter content and quality: Effects of fertilizer applications, burning and trash retention on a long-term sugarcane experiment in South Africa

The effects of crop residue management and fertilizer applications on soil organic matter (SOM) content and labile soil organic fractions were examined on a long-term (59 years) field trial under sugarcane situated at Mount Edgecombe, South Africa. Treatments at the site included pre-harvest burning with harvest residues removed (Bto), burning with harvest residues left at the soil surface (Bt) and green cane harvesting with retention of a trash blanket (T). Plots are either fertilized annually with N, P and K or unfertilized. Soil organic matter content in the surface 10 cm increased with increasing inputs of crop residues (Bto < Bt < T) and with annual fertilizer applications. There were no significant treatment effects on organic C or total N content below 10 cm but for microbial biomass C and N, readily mineralizable C and N, K2SO4-extractable and light fraction C, treatment effects were evident in the 10-20 and 20-30 cm layers. Changes in soil N content were more marked than those for organic C and, as a result, the C-to-N ratios of total SOM, the readily mineralizable fraction and the microbial biomass were all decreased by both increasing crop residue returns and fertilizer additions. By contrast, both crop residue and fertilizer inputs increased the microbial quotient since microbial biomass C was increased to a greater extent than total soil organic C. The results demonstrate that greater inputs of organic matter due to either increased returns of above-ground crop residues or increased deposition due to higher yields (induced by annual fertilizer applications) cause a proportionately greater increase in the size of labile pools of organic matter than in the total organic matter content. © 2002 Elsevier Science Ltd. All rights reserved.