Stacked conservation practices reduce nitrogen loss: A paired watershed study.

Combinations of best management practices (BMPs) are needed to achieve nutrient reduction goals in the Mississippi/Atchafalaya River Basin (MARB), but field results are crucial to encourage stacked adoption of BMPs. A paired catchment-scale study (2015-18) was done to assess the impact of (i) BMPs, (ii) precipitation patterns, and (iii) seasonality on nitrogen (N) export. Flow-weighted samples were collected and analyzed for total ammonia nitrogen (TAN), nitrate (NO-N), and total nitrogen (TN). Catchments Low-BMP 11 and High-BMP 12 had 27.6% and 87.6% areal coverage of BMPs, respectively. No significant difference (p > 0.05) in TAN concentrations was found between Low-BMP 11 (0.023 mg L) and High-BMP 12 (0.020 mg L). However, NO-N and TN concentrations were significantly higher (p < 0.05) at Low-BMP 11 (NO-N: 26.0 mg L, TN: 28.7 mg L) than at High-BMP 12 (NO-N: 8.8 mg L, TN: 9.2 mg L). Two precipitation factors that affected N export patterns were observed. First, N flushing could continue for several years after a drought as elevated NO-N concentrations were observed in 2015 (i.e., two years after the 2011-2013 drought). Second, higher annual N export was observed when more precipitation occurred during the pre-planting or early-growing season versus later periods. For both catchments, the highest 50% of flows were responsible for majority of the NO-N export. We estimated that 33-37%, 61-62%, and 82-85% of the NO-N loads occurred in the 90th, 75th, and 50th flow percentiles, respectively. As demonstrated in High-BMP 12, stacked BMP application effectively lowered NO-N and TN loads by 60.3% and 59.1%, respectively, relative to Low-BMP 11. Although 27.6% BMP coverage area in Low-BMP 11 was considered low for this study, this coverage area is higher than many other parts of the MARB. This research highlights the importance of joint efforts between landowners in a watershed to meet downstream water quality goals.