Lasting effects of soil health improvements with management changes in cotton-based cropping systems in a sandy soil

The soil microbial component is essential for sustainable agricultural systems and soil health. This study evaluated the lasting impacts of 5 years of soil health improvements from alternative cropping systems compared to intensively tilled continuous cotton (Cont. Ctn) in a low organic matter sandy soil. Our previous study (phase I) evaluated soil health microbial indicators (microbial biomass, community composition and enzyme activities) during 5 years (2003-2007) when more plant residue was returned to the soil by rotating cotton (primary cash crop) with grain sorghum (secondary cash crop) with and without a rye winter cover crop (Ctn-Rye-Sg and Sg-Ctn) or with a system that involved no cash crop and returned maximum biomass to the soil (3 times residue vs Cont. Ctn) with a sorghum x sudangrass hybrid with winter rye cover (SSd-Rye). The current study (phase II) addressed what happens to the microbial component (same microbial indicators) once the management is changed to more cotton production the following 3 years (i.e., cotton was planted 2 out of 3 years in SSd-Rye and was grown for 3 years in Ctn-Rye-Sg). During the first year of phase II (2008), all plots were planted in cotton, and higher cotton yields in SSd-Rye (108–150%) corresponded to microbial trends found in the original rotations (SSd-Rye > Sg-Ctn = Ctn-Rye-Sg). In regard to microbial indicators, the most significant trends with reintroduction of cotton and increase in tillage were a reduction in fungal FAME (fatty acid methyl esters) indicators with a lower ratio of fungi to bacteria and much lower AMF (15.1 to 3.98% of arbuscular mycorrhizal fungi), and less biogeochemical cycling potential of soil according to several enzyme activities (by as much as 50%). At the end of phase II (2010), Ctn-Rye-Sg and SSd-Rye were still found to have higher microbial biomass and enzyme activities (1.5 times both measurements except for phosphodiesterase) compared to continuous cotton. This study demonstrates the vulnerability of microbial communities in sandy soils, which requires agroecosystems that will support biomass incorporation (e.g., sorghum and cover crops) when possible in order to sustain essential functions and overall soil health.