Long term tillage, cover crop and fertilization effects on microbial community structure and activity: Implications on soil quality

Authors: MBUTHIA, LILIAN - University Of Tennessee; Acosta-Martinez, Veronica; DEBRYUN, JENNIFER - University Of Tennessee; SCHAEFFER, SEAN - University Of Tennessee; TYLER, DONALD - University Of Tennessee; ODOI, EVAH - University Of Tennessee; MPHESHEA, MOLEFI - University Of Tennessee; WALKER, FORBES - University Of Tennessee; EASH, NEAL - University Of Tennessee

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/16/2015
Publication Date: 10/1/2015
Citation: Mbuthia, L.W., Acosta Martinez, V., Debryun, J., Schaeffer, S., Tyler, D., Odoi, E., Mpheshea, M., Walker, F., Eash, N. 2015. Long term tillage, cover crop and fertilization effects on microbial community structure and activity: Implications on soil quality. Soil Biology and Biochemistry. 89(2):24-34.

Interpretive Summary: Reduced tillage, use of cover crops and balanced use of fertilizers are some of the conservation agriculture (CA) practices being promoted for the mitigation of soil erosion and greenhouse gases emissions, and improvement in soil and water quality, and crop productivity. The responses of the microbial community to soil management practices are key in soil quality evaluations. This is because their response is often associated with changes in soil organic matter (SOM) levels, greenhouse gases emissions and the accumulation and potential loss of nitrate by leaching. For these reasons, the replenishment of SOM and an increase in the abundance and activity of soil microbes are some of the indicators more directly attributed to improved soil quality under CA because they play a central role in governing key soil functions and properties such as nutrient cycling and retention, soil aggregation, soil aeration, water holding capacity, and root proliferation. While several studies have shown that reduced tillage, cover crops and N-fertilization result in changes in soil microbial structure and biochemical properties, there is still a paucity of information on the effects of these management practices and their interactions on soil microbial structure, activity and overall soil quality over the long-term (> 10 years) especially for low biomass cropping systems like cotton. The establishment of long-term cover crop studies is constrained by moisture limitations in regions, where cotton is produced in large quantities (e.g. Southern High Plains). In Tennessee, one of the main challenges is getting the cover crops established before the first frost. However, a greater soil moisture regime has allowed evaluating the effects of long-term (31 years) management practices (tillage, cover cropping and N-fertilization) on microbial community structure and activity based on microbial biomass C and N, microbial community composition (via fatty acid methyl ester (FAME) analysis) and microbial activity (microbial respiration and measurement of select enzyme activities of C, N and P cycling). Additional properties allowed the calculation of a soil quality index. This study brought a unique opportunity that provided vital information distinguishing the independent effects of each of these practices for low biomass cropping systems (continuous cotton production) where the integration of reduced tillage and cover cropping could help to compensate for the low biomass for this crop.

Technical Abstract: Reduced tillage, cover crops and fertilization are associated with greater microbial biomass and activity that are linked to improvements in soil quality, but their impacts vary widely with climate, soils and cropping systems. This study aimed to characterize the impact of long term (31 years) tillage (till and no-till), cover crops (Hairy vetch- Vicia villosa and winter wheat- Triticum aestivum, and a no cover control), and N-rates ( 0, 34, 67 and 101 kg N ha-1) on soil microbial community structure, activity and resultant soil quality calculated using the soil management assessment framework (SMAF) scoring index on a continuous cotton (Gossypium hirsutum) production research site located on a Lexington silt loam in West Tennessee. No-till treatments were characterized by a significantly greater (P < 0.05) abundance of Fatty Acid Methyl Ester (FAME) biomarkers associated with Gram+ bacteria, actinomycetes and mycorrhizae fungi compared to till. In contrast, saprophytic fungal FAME biomarkers were significantly less abundant (P < 0.05) under no-till treatments resulting in a lower fungi to bacteria (F:B) ratio compared to till. Key enzymes associated with C (ß-glucosidase), C & N (ß-glucosaminidase) and P cycling (phosphodiesterase) were significantly greater under no-till relative to till which was reflected in significantly greater (P < 0.05) soil C and N, extractable nutrients (P, K and Ca) and yields under no-till compared to till. Cover crop and N-rates mainly had an effect on the mycorrhizae fungi biomarker which significantly decreased (P < 0.05) with increasing N-rate and was significantly less (P < 0.05) under the vetch cover crop compared to wheat and no cover. Treatments under vetch showed a significantly higher metabolic functioning evident in the activity of ß-glucosaminidase (N-cycling) and basal microbial respiration, which was significantly greater (P < 0.05) under the vetch compared to wheat and no cover. Total C and N, and extractable nutrients (P, K and Ca) and yield increased with increasing N-rates with the greatest levels recorded under the highest N-rate. Consequently, the total organic carbon (TOC) and ß-glucosidase SMAF quality scores were significantly greater under no-till compared to till. Treatments under the vetch cover crop also had significantly greater TOC and ß-glucosidase quality scores compared to the wheat and no cover treatments that resulted in a significantly greater overall soil quality index (SQI). Our results demonstrate that long-term no-till and use of cover crops under a low biomass monoculture crop production system like cotton results in significant shifts in the microbial community structure, activity, and conditions that favor C, N and P cycling compared to those under conventional tillage practices. These practices also lead to higher soil quality and increased yields.