Quality and decomposition of black locust (Robinia pseudoacacia) and alfalfa (Medicago sativa) mulch for temperate alley cropping systems

Abstract

Black locust and alfalfa mulch quality were characterized over a 9-week period of decomposition. Both mulch materials were harvested with a forage harvester. The black locust material was harvested from trees annually coppiced for four years, the alfalfa from a preflorescent second cutting. Two identical, adjacent experiments were conducted, one on each mulch source. A randomized complete block design was used with four replications for this single factor experiment. Fifty grams fresh weight of both mulch types were placed in litter bags, made of 98% shade cloth, a woven plastic with holes approximately 1×0.5 mm. The treatments included two placements of the mulch material, surface and buried. Sampling took place on weeks 0, 1, 3, 6, and 9.

The incorporation of mulch materials appears to accelerate dry weight loss. From an initial dry weight of 44.25 g, the buried black locust mulch averaged 4.58 g, while the surface mulch averaged 7.41 g by week 6. The alfalfa mulch initially weighed 40.99g, with buried mulch averaging 1.75 g and surface mulch averaging 4.08 g by week 9.

Carbon concentration of alfalfa mulch increased with time in both mulch placements from 43.6% to 57.6% and 51.8% in surface and buried mulch, respectively. Black locust, with an initial carbon concentration of 46.9% increased to 55.9% for the buried mulch and to 53.89% in surface mulch. Nitrogen concentrations increased as well in buried mulch. Lignin concentrations increased dramatically over time for both species of mulch. Black locust lignin concentrations rose from 13.11% to 51.0% (buried) and 32.9% (surface); alfalfa mulch lignin concentrations rose from 6.67% to 31.4% (buried) and 47.7% (surface).