Abstract
In conservation tillage systems, at least 30% of the soil surface was covered by crop residues which generally contain significant amounts of nitrogen (N). However, little is known about the multi-seasonal recoveries of the N derived from these crop residues in soil-crop systems, notably in northeastern China. In a temperate agro-ecosystem, 15N-labeled maize residue was applied to field surfaces in the 1st year (2009). From the 2nd to 4th year (2010-2012), one treatment halted the application of maize residue, whereas the soil in the second treatment was re-applied with unlabeled maize residue. Crop and soil samples were collected after each harvest, and their 15N enrichments were determined on an isotope ratio mass spectrometer to trace the allocation of N derived from the initially applied maize residue in the soil-crop systems. On average, 8.4% of the maize residue N was recovered in the soil-crop in the 1st year, and the vast majority (61.9%-91.9%) was recovered during subsequent years. Throughout the experiment, the cumulative recovery of the residue N in the crop increased gradually (18.2%-20.9%), but most of the residue N was retained in the soil, notably in the 0-10 cm soil layer. Compared to the single application, the sequential residue application significantly increased the recovery of the residue N in the soil profile (73.8% vs. 40.9%) and remarkably decreased the total and the initially applied residue derived mineral N along the soil profile. Our results suggested that the residue N was actively involved in N cycling, and its release and recovery in crop and soil profile were controlled by the decomposition process. Sequential residue application significantly enhanced the retention and stabilization of the initially applied residue N in the soil and retarded its translocation along the soil profile.
Highlights
Conservation tillage can effectively improve soil quality through the input of fresh organic matter from crop residues [1,2]
The aboveground crop yields exhibited no significant difference between the Treatment 1 (T1) and Treatment 2 (T2) treatments in the identical year, the yields varied significantly throughout the experiment years (Table 1)
The crop residue applied on the soil surfaces generally undergoes slower decomposition than those incorporated into the tillage layer because of limited microbial accessibility, notably under lower temperature and precipitation conditions [14,18,29], resulted in a lower crop uptake of residue N in the 1st year in our experiment (Table 1)
Summary
The field experiment was initiated in the spring of 2009 at the National Field Observation and Research Station of Shenyang Agro-ecosystems (41°320 N, 122°230 E) in northeastern China. All necessary permits were obtained from the station for the in situ field research. Maize is the staple crop in the typical mono-cropping system, and this crop is sown at the end of April and harvested in late September in this region. The soil type of the experimental field is Alfisol doi:10.1371/journal.pone.0133437.g001. The weather at the site is typical of a temperate, humid, continental monsoon climate. During the experimental period (2009–2012), the mean annual precipitation (MAP) was 678 mm (Fig 1). The precipitation during the maize growing season (May-September) accounted for approximately 75% of the MAP. From 2009 to 2012, the temperature during the growing season varied slightly, with an average value of 21.2°C (Fig 1)
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