Long-term biomass removal in grasslands reduces plant and soil phosphorus, increases carbon:phosphorus stoichiometry, but does not lead to microbial phosphorus limitation

Abstract

Grassland management practices can disrupt the stoichiometric balance between available soil nutrients and microbial communities, thereby impacting ecosystem functioning. Using a long-term field trial, we investigated the effects of four contrasting mowing treatments with/without nitrogen (N) addition on the elemental concentrations and stoichiometries of soil, plant, and microbial biomass. Retaining biomass after mowing resulted in lower C:N and C:P in plant shoots (C:N = 18.8, C:P = 283.9) compared with never mowing (C:N = 24.5, C:P = 493.4). Similarly, the mean soil available C:N when retaining biomass after mowing (C:N = 2.68) was lower than the ratio in the never mown grassland (C:N = 3.44). Removing biomass after mowing strongly depleted soil available phosphorus (P) concentration, leading to significantly higher C:P (C:P = 16.10) than that in the never mown grassland (C:P = 3.85). There were no significant differences in soil available carbon (C) concentration among the treatments, indicating that shifts in the C:N and C:P ratios of available resources were largely due to differences in nutrient rather than C availability. Despite differences in the stoichiometry of available resources, microbial biomass C:N and C:P was similar among treatments and consistently higher than those for the available substrates. This indicates that soil microbes were unresponsive to varying nutrient availability across treatments, suggesting they may be C- rather than nutrient-limited. In this long-term trial, microbial stoichiometry was resilient to soil P depletion and insensitive to N addition. The components of agroecosystems (plants, soil, and soil microorganisms) can contrast in their stoichiometries. Managing one component’s nutrient limitation may have little influence with that of the other components.