Nitrogen response efficiency, nitrogen retention efficiency, and asymbiotic biological nitrogen fixation of a temperate permanent grassland site under different sward compositions and management practices

Abstract

Human activity had a huge impact on the global nitrogen (N) cycle in the last decades, mainly through an increase in the production of reactive N. In European grasslands, the increased use of N fertilizers was accompanied by increased mowing frequencies. This agricultural intensification made grasslands important sources of N losses and caused a decrease in biodiversity. Minimizing N losses and maintaining both soil quality and high yields at the same time is an important challenge in grassland management. This can be achieved through 1) an efficient retention of soil available N in the plant-soil system and 2) a high N response efficiency (NRE; i.e. a high plant biomass production per unit of plant-available N). The anthropogenic increase in the production of reactive N has been reviewed extensively. However, the comparison between anthropogenic N sources and biological N fixation (BNF) is difficult because existing data on (asymbiotic) BNF are incomplete and spatially unresolved. In the present study, our goal was to assess how management practices and sward functional diversity affect NRE, N losses, N retention efficiency and asymbiotic BNF in a temperate grassland. A three-factorial design was employed: three sward compositions (differing in species richness and the proportion of dicots and monocots), two mowing frequencies, and two fertilization treatments. NRE was calculated as the amount of harvested biomass per unit of plant-available N. Additionally, we developed an index that describes N retention efficiency as an ecosystem property and that relates N losses to gross N mineralization as an index for soil fertility. Asymbiotic BNF was measured on intact soil cores incubated separately in the field and in the lab using the acetylene reduction assay calibrated against direct measurements with 15N2 label. Fertilization was the dominant factor decreasing NRE and microbial N immobilization. Consequently, fertilization resulted in higher N losses and lower N retention efficiency. Intensive mowing partly reduced the high N losses following fertilization. Untreated control swards that have developed under decades of extensive management practices had the highest NRE and N retention efficiency. This pattern could be partly explained by complementary plant resource use. At the same time, it was also the result of the differences in microbial immobilization of ammonium which was the most important mechanism for an efficient N retention in the plant-soil system. N retention efficiency considers this pathway and thus, is a critical factor to consider when evaluating the sustainability of grassland management practices. The combined consideration of NRE and N retention efficiency provides a tool that accounts for biomass yield, gross N mineralization as a measure for soil fertility, and N losses to the environment. Thus, NRE and N retention efficiency are appropriate tools to evaluate the sustainability of grassland management practices which farmers employ to maximize profit. Our results showed that asymbiotic BNF can provide significant N inputs between 1.5 and 4.9 kg ha-1 yr-1 in the top 0.05 m of the soil profile. Fertilization with N and P decreased asymbiotic BNF. Potentially positive effects of a higher P availability were probably counteracted by negative effects of higher mineral N concentrations. Intensive mowing stimulated asymbiotic BNF most likely through an increase in rhizodeposition. Moreover, the 15N2 calibration showed that the conversion factor between acetylene reduction and N2 fixation can be considerably lower than the stoichiometric value of three. Lab-incubations under increased moisture and temperature conditions led to a strong increase in asymbiotic BNF. Previous estimates of asymbiotic BNF may thus, be substantially biased. Our study showed that asymbiotic BNF is of minor importance for intensively managed agricultural ecosystems. However, it may be an important pathway for BNF on a global scale that may substantially contribute to the amount of N fixed in the absence of human activities.