The effects of cutting frequencies at equal fertiliser rates on bio-diverse permanent grassland: Soil organic C and apparent N budget

Grasslands may mitigate rising levels of the atmospheric 'greenhouse gas' carbon dioxide by C sequestration in soil organic matter, depending on several environmental and management factors. The objective of this study was to examine the effects of long-term (19 years) different cutting frequencies (CFs), using equal amounts of mineral fertilisers (173 kg N, 40 kg P, and 150 kg K ha−1 year−1), on some sward and soil properties of previous extensively fertilised and utilised bio-diverse wet grassland. The impact of the study was directed towards the soil organic C and the apparent N budget. The CFs during the grassland experiment were cut at 2, 4, 6, 8, 10, and 12-weekly intervals. CF significantly affected the sward and the soil characteristics of the grassland. The average number of plants per plot was high at medium CFs and low at low CFs. The above-ground biomass yield was high at medium and at low CFs, and its N content was high at medium and at high CFs. Soil organic C stocks (SOCS), soil total N stocks (STNS), and soil bulk density showed significant interactive responses to CFs × soil depths (0 cm, 10–20 cm). Due to high amounts of symbiotically fixed N by legumes at higher CFs, the average above-ground biomass yields contained more N than was added with fertilizers. In contrast to the second soil layer, the first soil layer in those treatments was rich in SOCS and STNS. In treatment of 8-weekly intervals between cuts, the balance between N inputs and N taken-up by plants was closest to zero. Especially in those cases of low CFs (10 and 12-weekly intervals between cuts), the apparent N budget showed that N added with fertilisers was not fully removed from the system regarding above-ground biomass yields. Due to high SOCS and STNS in both soil layers it can be assumed that N surplus had been partly built into the soil organic matter. However, a part of N was uncontrollably lost from the soil–plant system. The obtained results highlight the complexity of the problem of C sequestration. In reality none of the treatments at the end of the experiment displayed everything, namely, high SOCS in both layers of the soil, low uncontrolled outputs of N from the system, high plant diversity and high yields of above-ground plant biomass.