Integrating Multiple Soil Quality Impacts from Brassica Cover Crops.
Ray R. Weil1, Guihua Chen1, Jill Dean1, Amy Kremen1, Lisa Stocking1, Yvonne Lawely1, Bahram Momen1, Sandra Sardanelli1, Inga Zasada2, John Teasdale2, and Stacy Williams1. (1) Univ of Maryland, College Park, MD 20742, (2) U.S. Dept. of Agriculture- Agricultural Research Service, Beltsville, MD 20705
Winter cover crops can substantially reduce N leaching, soil erosion and losses of sediment and P from cropland. However, farmers often do not perceive economic benefits, so despite 25 years of research and promotion, adoption of winter cover cropping on Maryland farms remains too low to meet environmental objectives. Brassica cover crops may provide a range of benefits sufficient to encourage their widespread use so both farmers and society would gain. We studied rapeseed (Brassica napus) cvs. ‘Humus' and ‘Dwarf Essex', oilseed radish (Raphanus sativus) cv. ‘Adagio', forage radish (R. sativus) cv. ‘Dikon', and a mustard blend (B. juncea and Sinapis alba). These cover crops are relatively little studied or used in eastern USA. They were compared to rye (Secale cereale), the region's most widely used and researched cover crop. All were planted in Aug-Sep and all but rapeseed and rye died in Dec-Jan from frost. Field experiments with corn/soybean rotations were begun in 2000 at two research stations and expanded in 2003 to four sites. Soils were Psammentic Hapludults with loamy sand A horizons or Aquic Hapludults with silt loam A horizons. In some cases the brassicas were mixed with rye or clover. Some experiments studied means of cover crop planting and termination. Also, numerous on-farm trials evaluated benefits and practicality under a diversity of cropping systems. We aimed to quantify seven interrelated impacts of these cover crops: 1) capture of residual profile N to reduce nutrient pollution (studied using 2 m deep soil cores and 90 cm deep tension lysimeters at three sites); 2) residue decomposition and conversion of residue N to plant-available mineral N (studied using a laboratory incubation as well as 30 cm deep soil cores taken monthly at four sites); 3) alleviation of subsoil compaction by cover crop root “bio-drilling” (studied at three sites by monitoring soil water above and below the compacted plow pan [Figure 1, right], obtaining minirhizotron root images during cover crop and cash crop growing seasons, and using Bohm core-break method to measure cash crop root distribution [Figure 1, left]); 4) conservation of soil water by residue mulching and enhanced infiltration (studied by monitoring soil water changes under growing cash crops); 5) suppression of plant parasitic nematodes; 6) enhanced free-living soil nematode community; and 7) suppression of weed growth and weed seed germination. Results suggest that rape, forage radish and oilseed radish are at least as effective as rye in capturing residual soluble N, removing >150 kg N ha-1 from upper 2 m of soil if planted by mid-Sep. Broadcast (aerial) inter-seeding of forage radish into corn or soybean canopies near senescence showed some promise, however biomass and root/shoot ratio were smaller than when drilled in Aug, so N recovery may not be as deep under inter-seeded conditions. Nitrate in leaching water at 70 cm was much lower under all cover crops compared to no-cover in March-May, but substantial NO3- appeared under dead forage radish on the sandy soil by mid April. We found no suppression of soybean cyst nematode (Heterodera glycines) or Trichodoridae in soybean. Bacterial feeding nematodes, especially dauer larvae, were more abundant after both radishes, but rape, rye and control plots had more fungal feeding nematodes. These changes in nematode community structure may be related to N dynamics and higher C:N ratios in spring-killed rape and rye. Forage radish showed by far the greatest weed suppression, leaving a nearly weed free soil in spring and also affecting weed seed germination. Yields of soybean and corn were significantly higher after forage radish or forage radish+rye than after no cover crop in most sites/years, but it is not certain which mechanisms were responsible.