In situ validation of fungal N translocation to cereal rye mulches under no-till soybean production

Authors: WELLS, M - North Carolina State University; REBERG-HORTON, S - North Carolina State University; Mirsky, Steven; Maul, Jude; HU, SHUIJIN - North Carolina State University

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2016
Publication Date: 7/21/2016
Citation: Wells, M.S., Reberg-Horton, S.C., Mirsky, S.B., Maul, J.E., Hu, S. 2017. In situ validation of fungal N translocation to cereal rye mulches under no-till soybean production. Plant and Soil. 410:153-165.

Interpretive Summary: In agroecosystems, plant litter is frequently recycled back through tillage or left on the surface. The plant litter can stimulate nitrogen immobilization, however the duration and magnitude of this process is affected by residue management. Both laboratory studies and work in forest ecoystems have shown that N can be translocated from soil into surface plant residues. However, we do not know the extent to which translocation of N occurs in agricultural systems. Therefore we conducted field experiments to determine the extent to which this occurs in cover crop-based soybean production. To monitor N-transfer between cereal rye surface residue, soybean tissue, and extractable soil inorganic N pools, a stable isotope 15N as (NH4)2SO4 (99.7 at. ) was injected below the soil surface in situ at a rate of 1 mg 15N kg-1 soil into (4.4 cm depth) buried steel frames (0.58 m2). A 36% increase in cereal rye mulch residue nitrogen was observed and appears to be fungal mediated due to increases in funal quanta in the untreated check plots. This work provides important contributions to define the cycling of nitrogen in heavy residue crop crop fields and will be of use to researchers testing nitrogen fate in cropping systems. Furthermore, this information will form the basis for farmer recommendations on fertilizer use and pest management in high residue cropping systems.

Technical Abstract: Grass cover crop shoots can be used as a mulch to suppress weeds. Physical control (light and temperature) of weeds appear to be the primary mechanism with chemical control (allelopathy) playing a smaller role. However, limitations of nitrogen availability to weeds in a soybean crop have been identified as another mechanism in which grass mulches inhibit weed performance. Fungal translocation of N from the soil to the surface mulch has been demonstrated in laboratory experiments, but this mechanism has not been documented under field conditions. Translocation of N into surface mulches may play an important role in weed suppression. To assess the presence of fungal mediated N immobilization, two separate (Single Injection Study and Multiple Injection Study) experiments were performed at the Caswell Research Farm in Kinston, NC (KINS) (35.273206° N, –77.623816° W), and at the USDA-ARS Beltsville Agricultural Research Center (BARC) in Beltsville, MD (39.03269° N, –76.928143° W). To monitor N-transfer between cereal rye surface residue, soybean tissue, and extractable soil inorganic N pools in a cover crop-based no-till soybean cropping system, a stable isotope 15N as (NH4)2SO4 (99.7 at.) was injected below the soil surface in situ at a rate of 1 mg 15N kg-1 soil into (4.4 cm depth) buried steel frames (0.58 m2). Treatments consisted of an aqueous solution with and without the fungicide Captan (50 W, wettable powder, 48.7% active ingredient, 2.1% N) every 2-weeks after soybean planting. The cereal rye mulches were reduced in N content at both BARC and KINS, at 15 and 50 kg N ha-1, respectively. Despite the N losses from the cereal rye tissues, there was detectable increase of 15N atom% by 10-15% in the cereal rye surface mulch tissue. Substantial upward movement of N was observed over time, however; only one sampling date showed significant impact of the fungicide treatment. Increases in 15N atom% in the surface cereal rye mulch further supports laboratory observations that soil inorganic N is fungal translocated into above ground mulches. At six weeks after soybean planting (averaged over locations), the non-fungicide treatment was 36% greater in 15N concentrations, providing evidence of microbial competition via fungal translocation as a mechanism. Throughout the season, soil extractable inorganic N was reduced by as much as 60% from the initial low of 10 kg N ha-1, where both the soybeans and cereal rye residues sequestered and immobilized considerable amounts of N. These findings support the microbial competitive nutritive weed suppression mechanism of high biomass cereal rye cover crop mulches during the critical weed free period of soybeans, thereby contributing to the weed suppression of the system.