Perennial plant cover controls soil nitrogen retention in managed current and former grasslands of the semi-arid western Great Plains

Background/Question/Methods

Nitrogen (N) loss via gaseous emissions, erosion, and leaching from fields and pastures is a widely recognized environmental problem for modern agriculture. The mechanisms and consequences of N loss are well studied in humid and sub-humid agricultural regions, but less so in semi-arid regions, where leaching is a rare occurrence and the dominant loss pathway is to wind erosion and gaseous emissions. These semi-arid grasslands cover 11% of Earth’s ice-free terrestrial surface and are widely used for agriculture, primarily supporting livestock grazing and dryland wheat production in the United States. We aimed to quantify the effects of different agricultural land uses on nitrogen retention in the semi-arid western Great Plains using a long-term ¹⁵N tracer experiment. We compared the effects of agricultural management on soil N retention with the effects of environmental heterogeneity, namely variation in climate and in soil depth, texture, and organic matter fraction.  Fifteen years after ¹⁵N tracer additions, we resampled grazed and ungrazed native shortgrass prairie, planted perennial grass, and fields of three rotation intensities of no-till dryland cropping, all in eastern Colorado, USA. 

Results/Conclusions

We found that perennial grass cover overwhelmingly controlled N retention or loss in these semi-arid ecosystems. Only about 20% of tracer N was retained in no-till crop fields, compared to almost 100% retention in adjacent planted ungrazed perennial grass fields, which were managed in a manner similar to Conservation Reserve Program lands.  The intensity of rotational cropping in the no-till fields (the number of successive crops in between fallows) had no effect on N retention. In the native shortgrass prairie, ungrazed prairie retained 50% of tracer N compared to 40% for grazed prairie. A larger proportion of tracer N was retained in 0-10cm soils than 10-30cm soils, and differences among management treatments were most apparent in the 0-10cm soil depth. Along with grazing, climate and soil texture are traditionally considered to exert control on soil organic matter content and N retention in semi-arid systems. However, our results indicate these controls are much less important than land use management and the presence of perennial grass cover for long-term N retention.