Primary Macronutrient Dynamics of Sunn Hemp (Crotalaria juncea L.) Residue in Different Mulching Strategies for Organic Tropical Cropping Systems.

Soil conservation and soil nutrient management pose unique challenges in tropical organic cropping systems where tillage is the primary means of field preparation and weed suppression. Cover crops, conservation tillage, and mulching are strategies that provide numerous ecosystem services, but are seldom integrated in practice into cropping system plans. The objective of this research was to evaluate the integration of cover crops and vegetable crops in reduced tillage and alternative mulching systems on cover crop residue nutrient dynamics. Experiments were conducted on St. Croix, USVI in 2015 and 2016 at two independent field sites. Trials began with the establishment of sunn hemp (Crotalaria juncea L.) in all experimental areas on October 16, 2015 and termination on January 11, 2016. Four treatments were arranged in a RCBD and replicated three times. Treatments included: 1) sunn hemp mulch (SHM), 2) sunn hemp mulch plus hay (SHM+hay), 3) sunn hemp mulch plus black landscape fabric (SHM+fabric) and 4) sunn hemp mowed and incorporated that served as a control plot (SH+none). Sunn hemp mulch retained in-situ using a no-till roller-crimper for termination. Peppers (Capsicum annum L.) were transplanted into treatments on January 14, 2016. Litter bags (10 cm²) containing fresh sunn hemp residue were placed within row- middles of each plot relative to the location of that treatment’s cover crop residue at time of pepper transplant and were retrieved from the field at two-week intervals for ten weeks. Sunn hemp residue retrieved from litter bags tissue were analysed to determine residue nutrient content.  Above-ground biomass of sunn hemp at termination did not differ between fields and measured 3,717 kg ha^-1 in field 1 and 4,367 kg ha^-1 in field 2. Following termination, total N content of sunn hemp residue was stable for weeks two, four, and six measuring 2.0, 1.8, and 1.9 percent N, respectively. Total N content decreased in weeks eight (1.6%) and ten (1.3%), representing a 33 percent decrease between weeks six and ten. In the residue incorporated treatment with no added mulch (SH+none) total N content was less than SHM+fabric in field 1 and the SHM in field 2. Total C was lower in incorporated residue (SH+none) compared to all other surface mulch treatments indicating increased decomposition and nutrient release rates during the sample period. Phosphorus content in SH residue was greatest at week six (0.2%), but declined at weeks eight and ten to 0.16 and 0.13 percent, respectively. Similar trends were seen with K; however, for each analysis, K was greater in SHM+fabric and SHM+hay treatments. Overall, the SHM+fabric and SHM+hay treatments had the greatest marketable pepper yields at 41,769 and 39,097 kg ha^-1 jalapeno peppers and 56,820 and 52,295 kg ha^-1 serrano peppers produced, respectively. Results indicate that the decomposition and subsequent release of primary macronutrients from SH residue are primarily influenced by time and treatment. Generally, the release of primary macronutrients from SH residue occur between six and ten weeks after cover crop termination, thus being available for improved crop productivity and increased farm revenue.