Dynamics of Soil Respiration and Microbial Biomass in Relation to Land‑Use Types and Legumes‑Enriched Crop Rotations

In a long‑term stationary experiment established in 1977 on soddy‑podzolic heavy loamy soil, the effects of continuous fallowing, continuous barley cultivation, and crop rotations with varying proportions of leguminous crops on soil agrophysical properties and ecophysiological indicators of organic matter activity were studied. It was found that the typical crop rotation for the Middle Urals region, namely, two clover (Trifolium pratense L.) fields and the application of cattle manure at 40 t/ha in the fallow field raised organic carbon content in the arable layer (0–20 cm) by 38.4% relative to the initial level over the 48-year period of the experiment. An increase in the proportion of leguminous crops in the crop rotation structure from 33.3% to 50% contributed to a rise in organic carbon (Corg) content by 13.9–29.1% relative to its initial value. An increase in total nitrogen content in the soil was observed as well: from 1050 to 1360 mg/kg without fertilizer application, and from 1130 to 1460 mg/kg under long‑term application of N60P30K60 for grain crops, depending on the percentage of leguminous crops in the rotation. The proportion of potentially mineralizable organic matter ranged from 2.0 to 3.7% of total carbon, depending on land management practices. The range of variation in basal respiration in the arable soil layer (0–20 cm) under different land uses without mineral fertilizer application was from 0.6 to 4.3 µg C–CO2/(g h). Mineral fertilizers did not have a significant effect on basal respiration. During the most favorable period in terms of temperature and moisture conditions in the typical crop rotation, an increase in microbial biomass up to 917.3 µg/g of soil was recorded. Furthermore, an increase in the proportion of legumes in crop rotations from 16.6 to 50% promoted an increase in microbial biomass carbon (Cmic) from 657.0–909.3 µg/g in unfertilized treatments and from 589.0–601.0 µg/g of soil under the application of complex mineral fertilizer. An increased proportion of legumes in rotations contributed to the enrichment of soil organic matter with microbial carbon and an increase in the proportion of Cmic in Corg up to 6.4%. The potential (microbial) flow of carbon dioxide from the soil of crop rotations with different levels of leguminous crops was 13.3–88.4 g CO2/(m2 day) in the topsoil, depending on the experimental options and the growing season.