Residue Quality and Decomposition Rate of Terminated Sunn Hemp Grown for Seed

The tropical legume sunn hemp (Crotalaria juncea L.) is primarily used a summer cover crop in southeastern vegetable crop production systems. Farmers value sunn hemp for its many benefits to the farming system including rapid growth, high biomass yield, efficient nitrogen (N) fixation, and weed and nematode suppression. Seed is imported from production areas in southern Africa at a cost of $7.00-9.00 kg^-1. Cultivars available for purchase are predominantly short-day varieties and consequentially fail to produce adequate seed in the normal period of summer cover crop production (six to eight weeks in Florida). The effects of cultural practices used to manipulate floral initiation and seed production have not been fully examined on biomass production, N sequestration and N mineralization. A 2-year field trial was conducted in the Organic Unit at the UF-IFAS Plant Science Research and Education Unit in Citra, FL to ascertain if managing sunn hemp for seed production would provide additional N benefits. The cultivar ‘Tropic Sun’ was seeded to 11, 28, and 45 kg ha^-1. In each density, 15 cm in length was cut from the main stem at three, four and five weeks after seeding to break apical dominance and encourage additional branching and flowering. Treatments were arranged in a randomized complete block design and replicated four times. Once half of each plot area was terminated at midbloom and the second half was terminated at seed maturity. Litter bags were prepared from sunn hemp residue and removed weekly for six weeks following termination by mowing. Plant height, biomass, tissue carbon and nitrogen, soil nitrogen, soil temperature and moisture, and crop carbon and nitrogen at termination and during decomposition comprise the scope of data collection.