Influence of Cover Crops and Fertility Management on Soil Health and Soil Microbial Community

Abstract

The southeastern United States has a long history of soil degradation due to intensive agriculture, and conservation agriculture practices aim to improve soil productivity. Conservation management practices include nutrient management, crop rotation, maintaining a soil cover, and conservation tillage. In order to measure the effect that management has on the soil, soil health indicators are used. Soil health indicators include analyses that measure the biological organisms in the soil, the soil chemical composition and nutrient content, and the physical structure of the soil. The objectives of this study were to evaluate the short and long-term effects of cover crops and soil fertility management on soil health indicators on Southeastern soil types. At the historic Cullars Rotation (est. 1911) in Auburn, Alabama, eight fertility and cover crop treatments were selected to evaluate their impact on soil health and the soil microbial community after 110 years of management. The treatments were complete fertility, no nitrogen (N), no winter legume cover crop, no N or cover crop, no phosphorous (P), no potassium (K), no lime, and no amendments or cover crop. Soil samples were taken from these treatments three times throughout the growing season from the 0-10 cm depth. Soil properties evaluated were soil moisture (θm), pH, soil texture, cation exchange capacity (CEC), plant available soil nutrient content with Mehlich-I extraction, microbial biomass carbon (MBC), soil respiration (Rs), soil organic carbon (SOC) and N, permanganate oxidizable carbon (POXC), autoclaved citrate extractable (ACE) protein, aggregate stability, and total bacteria gene copies with quantitative polymerase chain reaction (qPCR) analysis. The ‘complete fertility’ treatment was higher in SOC and POXC than all other treatments across all sampling times due to increased plant biomass production and therefore higher C inputs into the soil. Conversely, the ‘no amendment’ treatment preformed lowest in many of the soil health indicators measured including SOC, POXC, and ACE protein. Soil organic C and POXC did not show large responses to cover cropping, despite many reported responses of these indicators to conservation management in previous studies. Under N limited conditions, cover crops showed a response in ACE protein levels, but not when commercial N was applied in the rotation. No other soil health indicators showed a cover crop effect when inorganic N fertilizer was not applied. Aggregate stability was not correlated with any other soil health indicator nor was it affected by any treatment, and it may not be a viable soil health indicator for this soil type. Microbial biomass C, Rs, POXC, ACE protein, SOC, and total bacteria were all positively correlated to each other. This would indicate that these are effective soil health indicators in many cases and that each of these soil properties are related to one another. However, soil health indicators were not always reflective of soil fertility. For example, the ‘no lime’ treatment contained equivalent SOC and POXC as the ‘complete’ fertility treatment at some sampling dates due to reductions in microbial population. The ‘complete’ treatment had 2-3 times greater total bacteria and MBC than the low pH treatments (i.e., ‘no lime’ and ‘no amendments’, respectively), and this smaller microbial population reduced the amount of SOC being decomposed. When used to evaluate very low fertility treatments, some soil health indicators may be misleading, and a variety of indicators should be used to understand the complex soil dynamics that contribute to soil health and productivity. In order to determine the effect of cover crop monocultures and mixtures on soil health, cover crop experiments were established in the Coastal Plain and Tennessee Valley regions of Alabama and four years of data were collected at each location. Cover crops were incorporated into cotton (Gossypium hirsutum) and legume cash crop rotations, and the treatments included monocultures and two- and three-way mixtures of cereal rye (Secale cereale), crimson clover (Trifolium incarnatum) and forage radish (Raphanus sativus). Cash crop yield, cover crop biomass, SOC, POXC, aggregate stability, and soil strength (AUCC.I.) were evaluated. Cover crop biomass was variable from year to year and was dependent on planting date and weather conditions. In the Coastal Plain, treatments containing clover had on average 44% higher above ground biomass than those that did not have clover. At the northern Tennessee Valley location, the rye treatments performed slightly better than clover, but both rye and clover had much higher biomass than radish. In one site-year, a two-species mixture produced more biomass than both of its monoculture constituents. In the top 5 cm of soil, all cover crops with the exception of the radish monoculture increased SOC by 23% compared to the winter fallow treatment. In the 5-10 cm depths, rye-radish and rye-clover mixes increased SOC by 17% compared to the fallow. Similarly, some cover crop treatments were able to increase POXC compared to the fallow control. In the Coastal Plain, cover crop treatments had little effect on SOC and POXC due to the coarse and low organic matter soil type. In Tennessee Valley, the soil is finer-textured and can retain more soil organic matter than the coarser-textured Coastal Plain soil. Soil organic C and POXC were both highly correlated, and both of these indicators may be useful for determining the effects of cover cropping in some soil types. Aggregate stability did not show many meaningful differences at either location. Soil strength was highly variable with season, but it was affected four out of the eight site-years of this study. Treatments containing rye or clover decreased soil strength in the Tennessee Valley by 19% after four years of cover crop utilization. Differences in soil strength were also observed in the Coastal Plain, but they were inconsistent. In the Tennessee Valley, the rye monoculture and each 2-species mixture were able to increase cotton yield 25% compared to the no cover crop control. Conversely, there were no cover crop treatments that were able to increase cash crop yield in the Coastal Plain. Utilization of cover crops shows the potential to improve soil health and reverse the effects of soil degradation depending on the soil type and the cover crops used.