Nitrate behaviors and source apportionment in an aquatic system from a watershed with intensive agricultural activities

Abstract

Nitrate pollution in aquatic systems caused by intensive agricultural activities is a serious problem in the Sanjiang Plain. In this study, a dual isotope approach (δ(15)N-NO3(-) and δ(18)O-NO3(-)) was employed to identify potential nitrate sources (atmospheric deposition, AD; NO3(-) derived from soil organic matter nitrification, NS; NO3(-) derived from chemical fertilizer nitrification, NF; and manure and sewage, M&S) and transformation processes occurring in the Abujiao River watershed located in the Sanjiang Plain. The Bayesian model (stable isotope analysis in R, SIAR) was utilized to apportion the contribution of the potential sources. In this watershed, the nitrate concentrations in the surface water were low (mean ± SD = 1.15 ± 0.84 mg L(-1)), and were greatly influenced by precipitation and land use conditions during the two sampling periods (the high flow period, September; the low flow period, November). On the contrary, in the ground water, high NO3(-) concentrations were observed (7.84 ± 5.83 mg L(-1)) and no significant temporal variation in NO3(-) was found during the sampling periods. The sampled water δ(18)O-NO3(-) values suggest that the nitrification process was not the main N cycling process, because most of the measured δ(18)O-NO3(-) values were above the expected δ(18)O-NO3(-) from nitrification throughout the sampling periods. Both the chemical and isotopic characteristics indicated that the signs of de-nitrification were absent in the surface water. However, significant de-nitrification processes were observed in the ground water for all sample periods. Results from the SIAR model showed that source contributions differed significantly during the two sampling periods. During the high flow period, chemical fertilizers and soil N fertilizer equally contributed to the major sources of nitrate in the surface water. In contrast, manure and sewage sources dominated the source contribution during the low flow period (November). This study suggested that with the assessment of the behaviors and sources of NO3(-), effective nitrate reduction strategies and better management practices can be implemented to protect water quality.

Using Dual Isotopes and a Bayesian Isotope Mixing Model to Evaluate Nitrate Sources of Surface Water in a Drinking Water Source Watershed, East China

“…The largest proportion of M & S was due to the most inhabitants and domestic animals in this area. This result was in agreement with the analysis of organic nitrogen and TN in Section 3.1, and the source allocation pattern was similar to previous findings in other areas [36,37]. NS contributed the most (MPE 29%) in WSW during wet season, whereas M & S occupied the largest proportion (37%) in dry season.…”

“…It was attributed to the fact that ESW has relatively higher forests coverage (about 79.82%), where soils held more organics (tree leaves) and moisture that were more suitable for nitrification processes of microbial activities. Nitrification from soil nitrogen acted as dominant source to stream in forest areas were also found in other studies [36,47,48]. In MSW, the contribution of M & S was the highest in both wet and dry seasons (mean probability estimations (MPE) were 31% and 42%, respectively), followed by NS and NF (MPE about 20% and 18% during wet and dry seasons).…”

“…Thus, the denitrification in surface water can be ignored in the study area. The fact that denitrification seldom occurred in surface water was also found in many other studies [25,[34][35][36][37].…”

“…The source values of δ 15 N-NO 3 − and δ 18 O-NO 3 − for NP were measured from the precipitation samples in the watershed (as shown in Table 3 [8,[31][32][33][34][35][36][37][38][39][40]. It was assumed that the fractionation factors equal zero, as denitrification did not occur in surface water in the study area.…”

“…It should be noted that organic nitrogen productions from manure and sewage were much higher in MSW and ZSW as a result of intensive agriculture and livestock feeding activities. Other potential source isotopic values (M & S, NS, NF and NFA) were based on the relevant literature [8,[31][32][33][34][35][36][37][38][39][40], as shown by boxes in Figure 5. It is clear to see nitrate concentrations being affected by the five sources.…”