High retention of N P nutrients, soil organic carbon, and fine particles by cover crops under tropical climate

Abstract

Soil and water conservation is a major challenge for the development of sustainable agriculture. Leaching and runoff of soil nutrients and fine particles negatively impact soil fertility, soil quality, and contribute to surface water eutrophication and sediment accumulation. Such issues are especially critical in a tropical or subtropical region, such as in south Florida with high rainfall and shallow aquifer. So far, little is known about soil particle and nutrient retention by cover cropping under contrasting rainfall intensity. Here, a pot experiment was conducted with four cover crops and two simulated rainfall rates, a low rate of 5.8 mm day−1 and a high rate of 11.6 mm day−1, to study the transport of N, P, and fine soil particles down the soil profile. The cover crops were sunn hemp, velvetbean, cowpea, and sorghum sudangrass in contrast to weedy fallow as a control. Results show that under low rainfall rate, total N in leachates of 0.5–1.9 kg ha−1 for cover crops was much lower than 10.2 kg ha−1 for fallow. Under high rainfall rate, total N in leachates ranged from 3.4 to 16.1 kg ha−1 for cover crops and 8.8 kg ha−1 for fallow. Cover cropping with sunn hemp was the most effective at retaining soil nutrients because of its large plant biomass production. Indeed sunn hemp biomass accumulated 376 and 247 kg N ha−1 and 27 and 22 kg P ha−1 under high and low rainfall rates. We also found that soil organic carbon associated with fine soil particles in the root zone under sunn hemp and sorghum sudangrass was 5–7% greater than soil organic carbon in fallow. This study demonstrates that growing sunn hemp and sorghum sudangrass cover crops can conserve soil fine particles and nutrients against leaching and reduce adverse impacts on surface water.