Plant residue quality indices: a methodological approach for evaluating summer cover crops under tropical climates

The success of conservation agriculture in tropical environments depends on crop rotations that  ensure adequate quantity and quality of phytomass for persistent soil cover. The objective of this study was to evaluate biomass decomposition and nitrogen (N) release by summer cover crop species, employing distinct indices to assess their plant residues in terms of accumulated N release (NAR) and remaining dry matter (RDM). The experiment was conducted on a “Nitossolo Eutrófico” at the “Luiz de Queiroz” College of Agriculture, in a randomized block design with five treatments and four replicates. Treatments consisted of plant residues from sunn hemp (Crotalaria juncea L.), jack bean (Canavalia ensiformis (L.) DC.), Congo grass (Urochloa ruziziensis L.), pearl millet (Pennisetum glaucum L.), and maize (Zea mays L.). Biomass decomposition and N release were assessed using nylon litter bags, with samplings conducted at 15, 30, 60, and 90 days after crop termination. The legume species exhibited the highest N release rates, while among grasses, pearl millet had the highest NAR and the greatest RDM after 90 days. A significant linear correlation was found between the Plant Residue Quality Index (PRQI) and the rates of biomass decomposition and N release. At the same time, the Residual Biomass Quality Index (RBQI) identified tropical forages as more efficient in balancing NAR and RDM throughout the experimental period. These findings can support cover crop selection in tropical rotations aiming at synchronized N release and persistent soil cover.