No abstract

Rainfed agriculture is one of the most common farming practices in the world and is vulnerable to global climate change. However, only limited studies have been conducted on rainfed agriculture, mainly using low-frequency manual techniques, which caused large uncertainties in estimating annual N2O emissions. In this study, we used a fully automated system to continuously measure soil N2O emissions for two years (April 2017 to March 2019) in a typical rainfed maize field in Northeast China. The annual N2O emissions were 2.8 kg N ha−1 in year 1 (April 2017 to March 2018) and 1.8 kg N ha−1 in year 2 (April 2018 to March 2019), accounting for 1.9 and 1.2% of the nitrogen fertilizer applied, respectively. The inter-annual variability was mainly due to different weather conditions encountered in years 1 and 2. A severe drought in year 1 reduced plant N uptake, leaving high mineral N in the soil, and the following moderate rainfalls promoted a large amount of N2O emissions. The seasonal pattern of N2O fluxes was mainly controlled by soil temperature and soil nitrate concentration. Both soil moisture and the molar ratio of NO/N2O indicate that N2O and NO were mainly derived from nitrification, resulting in a significant positive correlation between N2O and NO flux in the intra-rows (where nitrogen fertilizer was applied). Moreover, we observed that the N2O emissions during the freeze–thaw periods were negligible in this region for rainfed agriculture. Our long-term and high-resolution measurements of soil N2O emissions suggest that sampling between LST 9:00 and 10:00 is the best empirical sampling time for the intermittent manual measurements.

Measuring the nitrogen isotope compositions (δ 15 N) of nitric oxide (NO) from different sources helps to quantify the relative contributions of atmospheric NO x. Soil is one of the most important sources of atmospheric NO x , but only limited measurements on the δ 15 N of soil-emitted NO exist, hampering our ability to partition sources to air pollution. Here we conducted soil incubations to measure the δ 15 N-NO under defined aerobic or anaerobic conditions, favoring either nitrification or denitrification. Soils were collected from seven sites spanning three ecosystems in northern China (two agricultural, two forest, and three grassland sites). We found that the δ 15 N-NO and their associated N isotope fractionations were significantly different between anaerobic and aerobic conditions in seven soils. Under aerobic condition, the δ 15 N-NO ranged from −62‰ to −50‰ (averaged −56 ± 4‰), being significantly more negative (by 23‰) than those under anaerobic condition (−45‰ to −23‰, averaged −33 ± 7‰). The apparent N isotope fractionation for NO production under aerobic condition (15 ε aerobic ¼ 61 ± 3‰) was significantly higher (by 26‰) than under anaerobic condition (15 ε anaerobic ¼ 35 ± 6‰), with a small variability among ecosystem types. Our study demonstrates that the δ 15 N-NO from different soils are very different from fuel combustions (mainly from 0‰ to +20‰), supporting that measuring 15 N is a useful tool to partition the contributions of soil NO to atmospheric NO x. Our results also imply δ 15 N-NO produced by nitrification and denitrification distinctly different, as these two processes are dominant processes producing NO under aerobic and anaerobic conditions, respectively. Plain Language Summary Nitric oxide (NO) affects the atmosphere chemistry and NO itself a key component of air pollution and a precursor to other air pollutants like particulate matter and ozone. To reduce the air pollution, it is crucial to identify the atmospheric NO sources. Measuring 15 N natural abundance has been considered as a promising tool to identify different sources, but the measurement on δ 15 N for soil-emitted NO is limited, adding to the large uncertainties on NO partitioning. Here we measured the δ 15 N-NO emitted from three ecosystem types across seven sites (two agriculture, two forest, and three grassland sites) under defined anaerobic or aerobic conditions. We found that soil-emitted NO was 15 N-depleted (−62‰ to −23‰) related to the source from fossil fuel combustion (0-20‰), suggesting that δ 15 N can be used to separate these two sources. We also found that the δ 15 N-NO under aerobic condition (nitrification prevailing, −62‰ to −50‰) was much lower than under anaerobic condition (denitrification prevailing, −45‰ to −23‰), which provide a useful tool to better understand the relative contributions of different sources of NO. In sum, soil-emitted NO has their own different isotope fingerprint, depending on the soil oxygen conditions, and can be used to partition sources.