Nitrogen retention effect of riparian zones in agricultural areas: A meta-analysis

Nitrogen cycling in riparian zones is a complex process. Few studies have comprehensively explored the physicochemical and biological removal of nitrogen in riparian zones. By summarizing 123 studies published between 1980 and 2020, we systematically review nitrogen removal from surface runoff, groundwater, plant uptake and microbial degradation in soil, in an attempt to comprehensively outline the nitrogen removal process in riparian zones. The results indicate that nitrogen reduction with buffer widths showed a one-phase exponential decay trend in both surface runoff (R2 = 0.36) and underground water (R2 = 0.16). At a width of 10-15 m, the average nitrogen reduction in surface runoff and underground water reached approximately 79 and 59%, respectively. In the investigation of microbial and plant nitrogen removal, denitrification rates and nitrous oxide (N2O) emissions rates increased by 25 and 94%, respectively, in 2010-2020 compared with 2000-2010. The nitrogen uptake, denitrification, and N2O emissions efficiency of forests were notably higher than those of grass. Warm weather is beneficial to denitrification and N2O emissions; however, nitrogen uptake by plants showed a downward trend over the past 20 years. Environmental factors (e.g., climate, vegetation, water quality and hydrology, topography, and soil) showed notable and sensitive impacts on nitrogen retention and the microbial nitrogen cycle. In addition, legacy nitrogen had a long-term effect on waterbodies and may require an extended period to deplete and recover. This meta-analysis provides a comprehensive and scientific basis for understanding retention effects of nitrogen in riparian zones of agricultural areas.

Automated summary

The study systematically reviewed 123 studies on nitrogen removal in riparian zones published between 1980 and 2020. It found that nitrogen reduction showed an exponential decay trend in surface runoff and groundwater, with higher reduction rates at buffer widths of 10-15 meters. Forests had higher nitrogen uptake, denitrification, and N2O emissions efficiency compared to grasslands, and warm weather was beneficial for denitrification and N2O emissions.