Effects of Catch Crops Cultivated for Green Manure on Soil C and N Content and Associated Enzyme Activities

The influence of catch crop (field pea) management and the time of its application [plots with autumn (A.I.) or spring (S.I.) biomass incorporation vs. treatments without catch crop (C) use] on the activity of five soil enzymes associated with C- (CMC-cellulase—CEL, β-glucosidase—βG, invertase—INV) and N-cycling (urease—UR, nitrate reductase—NR), the content of mineral N, and the content of microbial biomass C and N (MBC, MBN) were evaluated in a 3-year experiment (2009–2011). Field pea was sown in the first half of August and the biomass was incorporated into the soil in the second half of October in 2008, 2009, and 2010 or left as a mulch during the winter and mixed with the soil in spring. The enzymatic and microbial properties were determined in soil samples collected from spring barley experimental plots four times a year (2009, 2010, and 2011): in March (before sowing of barley), in May (during the tillering phase), in June (during the shooting phase), and in August (after barley harvesting). Neither the catch crop management nor the sampling date had any effect on the content of total organic carbon (CORG) or total nitrogen (Nt). The incorporation of filed pea biomass significantly enhanced the soil mineral N content (up to 30%), as well as the microbial and enzymatic variables, compared to the control soil. The spring use of field pea biomass significantly increased the content of microbial biomass C (2009 and 2011) in contrast to autumn incorporation. On the other hand, the biomass-N and the activity of the studied enzymes did not reveal statistically significant changes (or the results were inconsistent) as regards the time of catch crop biomass incorporation. The assessed variables (except for CORG, Nt, microbial biomass N) showed significant seasonal variability, but the changes were not clear or associated with a specific property. However, we found one similarity; the majority of the determined variables were the highest in May and/or June. Our data confirmed that catch crops play a key function in the management of agroecosystems. Plant biomass incorporated into soil is a source of organic matter, which increases nutrient concentrations and enhances soil biological activity. Because the time of catch crop application did not reveal consistent changes in the studied properties, both spring and autumn applications can be recommended as a suitable practice in modern agriculture.