Stoichiometry of soil organic matter and microbial functional traits altered after 5 years of maize monoculture

Abstract

The long-term transformation of organic matter in agroecosystems is modulated by soil properties and functional traits of plants and microorganisms, and is an indicator of soil quality. We studied how contrasting soil texture (loamy vs sandy) and two plant genotypes (a wild type, WT and a mutant deficient in root hairs, rth3) altered organic matter through microbial functioning in a field experiment after 5 years of maize monoculture. Our hypotheses were that (1) loamy rather than sandy soil will promote a larger OM storage; and (2) root-hairs, entailing a higher root-soil interface and water retention in the rhizosphere, boosting microbial processes and OM fluxes, will increase OM content not only in the rhizosphere but in the long-term, also in the bulk soil. To address these hypotheses, plots were filled with homogenized soil, being sandy soil a mix of 16.7 % loam with quartz sand, and the two maize genotypes. After 5 years of monoculture, soil was collected at the early maize growth stage of BBCH19, in the first 20 cm and the 20 to 40 cm depths. We determined stoichiometric ratios of total (TOC/TN) and labile (DOC/DON) fractions of soil organic matter, and microbial eco-physiological indexes related to OM transformation and sustainability, i.e. respiration-to-biomass ratio, qCO2, and microbial-to-total organic C ratio, Cmic:Corg.  The 5- and 7-times larger C supply in loamy vs sandy soil, for WT and rth3, respectively was explained by 40% more efficient C metabolism, i.e. less C losses through respiration per biomass unit, by 90% lower specific labile C content (DOC:Cmic ratio) and by 60% faster microbial turnover (µmax-1) in the former. As the TN content was only 3.1 and 4.2 times larger in loamy than sandy soil for WT and rth3 respectively, the resulting C:N ratio was 1.7- times greater for loamy than sandy soil, indicating more unbalanced stoichiometry in the former for both genotypes. Remarkably, the C and N content increased 20 and 36 %, respectively, after 5 years of maize monoculture, in the plots under root hair-deficient mutant than under wild type maize resulting in significantly larger C storage at rth3-plots in loamy soil. This may be explained by the reported higher exudates production by rth3 under the field conditions. Soil depth up to 40 cm did not show big differences in the capacity for C storage, likely due to similar root density in both depths. Concluding, loamy soil showed a higher capacity of C storage through microbial turnover and sandy soil demonstrated larger C losses through DOC compounds and respiration after 5 years of maize monoculture. Finally, carbon availability was not the limiting factor in sandy soil for microbial growth, but rather other factors such as soil water holding capacity, or OM fluxes from rhizosphere to bulk soil.