Excessive nitrogen (N) fertilizer input has become a common phenomenon among most farmers in the winter wheat–summer maize rotation system of north-central China, and has resulted in low nutrient use efficiency and environmental pollution. Controlled-release urea (CRU) is proposed as a solution to excessive fertilization because CRU achieves high yields and reduces N losses. Therefore, CRU mixed with normal urea at rates based on the Nutrient Expert (NE) system was used as fertilizer in a 4-year field experiment to test the preference in crop yields, economic benefits, nitrogen use efficiencies, and N losses. The following fertilizer treatments were established: local farmers’ practices (FP); normal urea fertilizer at the rate recommended by the NE system (NE); mixed CRU and normal urea at ratios of 60:40 (CRU1) and 75:25 (CRU2) based on the NE system; and 80% of the recommended N rate of the NE, CRU1 and CRU2 treatments (80%NE, 80%CRU1 and 80%CRU2). The results showed that, compared with the NE treatment at the same application rate of N, mixed CRU and urea increased yields and net benefits while reducing N loss. The application of CRU at 60% for maize and 75% for wheat had the best overall effects. Compared with FP, the average grain yield, recovery efficiency of N fertilizer and net benefits increased by 8.5%, 10.9% and 11.3%, respectively, for maize with CRU1, and increased by 4.5%, 15.1% and 10.3%, respectively, for wheat with CRU2. Furthermore, mixed CRU and urea at the recommended N rate significantly reduced N loss from 38.5% to 40.3% but increased soil NO₃⁻-N and NH₄⁺-N contents at 0–30 cm, although opposite results (NO₃⁻-N) were observed deeper in the soil (30–90 cm). In the treatments 80%CRU1 and 80%CRU2, the maize yield and overall economic benefits were equivalent to those in the FP treatment, but apparent N loss was significantly reduced. Thus, these results confirmed that the combination of the CRU and the NE system for winter wheat–summer maize in north-central China is efficient and valuable, and has the potential to improve yield, nitrogen use efficiency and net benefit with low N losses.

Aims Terrestrial carbon (C) sequestration is derived mainly from plant photosysthetically-fixed C deposition but soil organic C (SOC) content in saline soils is generally low due to low deposition of C from restricted plant growth. It is important to explore the effects of soil salinity on the allocation of photosynthetically-fixed C to better understand C sequestration in saline wetland soils. Methods We conducted a pot experiment in which reed (Phragmites australis) was grown in a low salinity (LS) soil and a high salinity (HS) soil from the Yellow River Delta under flooded conditions. The allocation of photosynthetically-fixed C into plant tissues, SOC, dissolved organic C (DOC), microbial biomass C (MBC), particulate organic C (POC), and mineral-associated organic C (MAOC) was determined using a ¹³C pulse-labeling method after four labeling events during the 125-day-long reed growing season and destructive sampling immediately at the end of six hours of pulse labeling (end 6-h) and on the final harvest day (final day). Results In most cases soil salinity, reed growth stage, or reed biomass significantly (P < 0.05) affected the deposition of photosynthetically-fixed C into the plant-soil system. At all four pulses at end 6-h the high salinity soil had significantly (P < 0.05) lower percentage net assimilated ¹³C in the roots and significantly higher (P < 0.05) percentage net assimilated ¹³C in the soil than did the low salinity soil. At both end 6-h and on the final day the high salinity soil had significantly (P < 0.05) lower SO¹³C, and significantly (P < 0.05) higher DO¹³C/SO¹³C ratio than the low salinity soil except for pulses 3 and 4 on the final day. The majority of photosynthetically-fixed C in soil was deposited into MAOC pools and >80 % of deposited SO¹³C was present as MAOC in the high salinity soil due to its significantly (P < 0.05) higher clay content compared with the low salinity soil. Conclusions Soil salinity affected the allocation of photosynthetically-fixed C in the plant-soil system, and soil texture altered the allocation of rhizodeposition C in different soil particles.

A maize pot experiment was conducted to compare the difference of N distribution between bulk and rhizospheric soil after chemical fertilizer with or without soil straw amendment at an equivalent N rate using a ¹⁵N cross-labeling technique. Soil N pools, maize N and their ¹⁵N abundances were determined during maize growth. The urea plus straw treatment significantly (p < 0.05) increased the recovery of urea N in soil and 26.0% of straw N was assimilated by maize. Compared with urea treatment in bulk soil, urea plus straw treatment significantly (p < 0.05) increased the concentration and percentage of applied N as dissolved organic N (DON) and microbial biomass N (MBN) from milk stage to maturity, increased those as particulate organic N (PON) and mineral associated total N (MTN) throughout maize growth and decreased those as inorganic N (Inorg-N) from the eighth leaf to the silking stage. Compared with bulk soil, rhizospheric soil significantly (p < 0.05) decreased the concentration and percentage of applied N as PON and increased those as Inorg-N and MTN in both applied N treatments from the silking stage, and significantly (p < 0.05) decreased the concentration and percentage of applied N as microbial biomass N (MBN) in the urea plus straw treatment. Overall, straw N was an important N source and combined application of chemical fertilizer with straw increased soil fertility, with the rhizosphere regulating the transformation and supply of different N sources in the soil–crop system.

Fertilizer application is important to achieve sustainable agriculture. However, it remains unclear about the effects of long term fertilization on C and N immobilization as well as C/N ratios in soil aggregates at different depths. Samples taken at depths of 0 to 40 cm from dryland red soil subjected to long-term fertilization were analyzed. Four treatments were involved in the long term fertilization including no fertilizer (control), chemical fertilizer applied at two different rates, and manure combined with chemical fertilizers (MNPK). The C and N concentrations in the soil aggregates of different sizes were significantly higher (P < 0.05) and the C/N ratios in the particulate organic matter were significantly lower (P < 0.05) for soil to 20 cm deep for the MNPK treatment than for the other treatments. ANOVA indicated that the C and N concentration and C/N ratios in different sizes of aggregates significantly varied with soil depth (P < 0.05). Microaggregates contained most of the C and N, and the C/N ratios for silt–clay particles in macroaggregates were 1.37 unit (ranging − 0.25 to 2.44) lower than for other soil particles with diameters < 53 µm. The C and N contents in aggregates of different sizes increased as the C input rate increased to a depth of 40 cm because of the fertilization practices. Overall, both increased C input and deep application of C sources promoted the storage of C and N in microaggregates, which in turn increased C and N sequestration in dryland red soils.