The study aimed to explore effects of different fertilization on paddy soil aggregate characteristics and potassium distribution under rapeseed-rice rotation and winter fallow by the located experiment test. It was hoped that the study would provide a basis for the sustainable utilization of potassium resources in paddy-upland rotation areas of Southern China.

Based on the location experiment of potassium fertilizer in different rotation patterns started in 2016, five treatments were selected, including CK(F) (no fertilization and winter fallow), NPK(F) (NPK fertilizer and winter fallow), NPK(R) (NPK fertilizer and winter rapeseed), NPK (SR+R) (NPK fertilizer with rice straw returning and winter rapeseed), and NP50%K (SR+R) (50% reduction of K fertilizer with rice straw returning and winter rapeseed). The soil samples were collected from the 0-20 cm soil layer of each treatment after rape harvest of the third year for analysis of physical and chemical properties, such as soil aggregates distribution, and exchangeable and non-exchangeable potassium content variation in soil aggregates. The stability of soil aggregates and the distribution of potassium in aggregates were further investigated.

The soil aggregate fractions of all treatments was the highest in <0.053 mm. Compared with NPK(F), NPK(R) increased the proportion of aggregates of 1-2 mm, 0.5-1 mm and 0.25-0.5 mm by 26.2%-82.6% under the same fertilization treatment. Also the stability of soil aggregates was increased, the proportion of aggregates of >0.25 mm, mean weight diameter (MWD), and geometric mean diameter (GMD) were significantly increased by 30.6%, 31.2% and 82.0%, respectively. Under rapeseed-rice rotation, the proportion of aggregates of >2 mm was increased under NPK(SR+R), which was 69.7% higher than that under NPK(R). The exchangeable potassium content in soil aggregates decreased with decreasing particle size under all treatments. NPK(SR+R) significantly increased exchangeable potassium content in all aggregate fractions by 22.2%-46.0% compared with NPK(R) under rapeseed-rice rotation. NP50%K(SR+R) significantly reduced the exchangeable potassium content in aggregates of >0.5 mm by 19.4%–20.6% than NPK(SR+R). Compared with NPK(F), three fertilization treatments under rapeseed-rice rotation all reduced the non-exchangeable potassium content in aggregate fractions. Under all treatments, the contribution rate of different fractions of soil aggregates to the bulk soil in potassium was the highest in <0.053 mm. Compared with NPK(F), NPK(R) significantly increased the contribution rates of 1–2 mm and 0.5–1 mm aggregates to bulk soil in potassium by 82.6%, 52.1% (exchangeable potassium) and 105.5%, 36.9% (non-exchangeable potassium), respectively.

The rapeseed-rice rotation could increase MWD, GMD, macroaggregate proportion and the contribution rate of macroaggregate to the bulk soil in potassium and improve soil structure. Under this rotation pattern, the chemical fertilizer combined with rice straw returning could increase the exchangeable potassium content in all aggregate fractions, which improved the potassium supply in paddy soil. However, the rapeseed-rice rotation reduced the content of non-exchangeable potassium in soil aggregates due to high potassium demand, and the input of potassium fertilizer should be appropriately increased.

Spatio-temporal variations and driving factors of fertilizer contribution rate were conducted for the paddy soil in China, so as to provide a theoretical basis for the sustainable development of rice and scientific recommended application of fertilizers in rice cultivation areas.

Data was cited from a total of 338 long-term paddy soil monitoring sites located in rice-producing provinces of China for this study. The effects of fertilizer contribution rate for cropping system, climate, regions, soil texture and its driving factors were evaluated.

In the past 30 years (1988-2017), the fertilizer contribution rate for rice increased first and then levelled off in China in the condition of same rice variety and cultivation techniques both in fertilization and none-fertilization plots. The variable range of fertilizer contribution rate for rice was from 41.20% to 51.89%. The fertilizer contribution rate for rice was 38.58%-55.49%, 41.96%-51.05%, and 42.34%-53.43% in single, double rice, and rice and other cropping systems, respectively. After 30 years, the fertilizer contribution rate for rice from high to low was as follows: Southwest China (55.82%), Middle of Yangtze River (46.73%), North China (46.27%), North east China (45.90%), South China (45.83%) and Lower of Yangtze River (44.25%). According to the fitting equation, it was found that the stable fertilization years of fertilizer contribution rate for rice was 15.2 a, 18.5 a, 19.0 a, 15.3 a, 15.3 a and 14.5 a in the Northeast China, North China, Southwest China, Middle of Yangtze River, Lower of Yangtze River and South China, respectively. The fertilizer contribution rate for rice in subtropical monsoon (49.23%) was much higher than temperate monsoon (45.90%) and tropical monsoon (34.57%). The fertilizer contribution rate for rice with different soil texture was 43.25%-64.80% for clay soil, 40.65%-48.46% for loam soil, and 26.20%- 45.98% for sand soil. Among the fertilization factors, nitrogen and phosphate fertilizer were the main factors affecting fertilizer contribution rate. Among the climatic factors, frost-free period, mean annual rainfall and mean annual temperature were the main factors and soil organic matter was the most important one among the soil factors.

The fertilizer contribution rate for rice was increased first and then levelled off in 1988-2017, and there was different for fertilizer contribution rate for rice under different rice cropping systems, regions, climate and texture. Nitrogen and phosphate fertilizers were main factors of fertilizer contribution rate for rice in China. Moreover, the soil organic matter in paddy soil was the most important to fertilizer contribution rate for rice in soil fertility indexes.

The effects of long-term fertilization on the distribution of organic carbon (OC) and total nitrogen (TN_OIC) of organo-mineral complexes in red paddy soil were studied, so as to provide a basis for long-term management and cultivation of soil fertility in red paddy soil.

The red paddy soils in long-term fertilization experiment were studied (Since 1984), which included four treatments: no fertilization (CK), inorganic P and K fertilizers (PK), inorganic N, P and K fertilizers (NPK), and NPK plus manure (NPKM, 70%NPK plus 30% manure). Soil samples of 0-20 cm soil layer were collected, and the distribution of organo-mineral complexes at each particle level (<2 μm, 2-10 μm, 10-20 μm, 20-50 μm, and 50-250 μm) was analyzed. The effects of fertilization on the content, storage of organic carbon and total nitrogen as well as the carbon-nitrogen ratio (C/N) were discussed. The effects of OC and TN_OIC contents in organo-mineral complex on contribution rate of SOC and TN contents in red paddy soil were investigated too.

Compared with the CK treatment, the fertilization treatment significantly increased the particle size ratio of 20-50 μm and decreased the particle size ratio of <10 μm. Compared with other fertilization treatments, NPKM treatment increased the proportion of 50-250 μm grain size complex more significantly. Different fertilization treatments had different effects on the content of OC and TN_OIC in grain size. Compared with PK, the content of OC and TN_OIC in grain size from 50 μm to 250 μm under NPK treatment increased 36.3% and 80.6%, respectively. Compared with NPK, the content of OC and TN_OIC in 50-250 μm granular complex increased by 35.4% and 19.5% under NPKM treatment, respectively. The OC and TN_OIC storage of the organic and inorganic complexes were mainly distributed at the 10-20 μm particle level. And fertilization significantly reduced the storage of <2 μm particle-level complex OC and TN_OIC, but increased the storage of OC and TN_OIC at 20-50 μm particle-level. Compared with PK treatment, the OC storages of <2 μm and 50-250 μm particle-level complexes treated by NPK increased by 18.5% and 31.2%, respectively, and the storages of TN_OIC increased by 18.8% and 73.7%, respectively. Compared with NPK, the NPKM treatment reduced the OC and TN_OIC storages of the <2 μm particle-level complex by 25.6% and 27.4%, respectively, while OC and TN_OIC storages of 50-250 μm particle-level complex increased by 56.3% and 38.6%, respectively. Fertilization significantly influenced the C/N ratio of 50-250 μm particles. Compared with PK, the C/N ratio of 50-250 μm fraction treated with NPK decreased by 24.6%; compared with NPK, the C/N ratio of 50-250 μm fraction treated with NPKM was increased by 13.4%. Fertilization significantly affected the contribution rate of organo-mineral complexes of each particle size to SOC and TN content. Compared with PK, the contribution rate of NPK treatment 50-250 μm particle-level complex to SOC and TN increased by 17.4% and 47.4%, respectively. Compared with NPK, the contribution rate of NPKM treatment 50-250 μm particle to SOC and TN were reduced by 39.5% and 32.8%, respectively.

In red paddy soil, the long-term fertilization promoted the formation of large-grain organo-mineral complexes. The nitrogen input in the chemical fertilizers significantly increased the grain-level organic carbon, total nitrogen content and storage of the granular grade of 50-250 μm. The organo-mineral combined application was conducive to increasing the soil organic carbon content and the proportion of large-grain organo-mineral complexes, which was helping to preserve the inorganic nitrogen fertilizer application. Therefore, organic and inorganic compound application was an effective measure for long-term fertilization management of red paddy soils.

This study aimed to explore the spatial-temporal variation characteristics and influencing factors of relative yield gap of wheat and maize in China during the past 15-20 years and the response of relative yield gap to nitrogen fertilizer under different soil productivity levels, so as to provide a theoretical basis for rational application of nitrogen fertilizer and the realization of high and stable yield of wheat and maize.

Based on the long-term monitoring database, the difference of wheat and maize yield between fertilized area and non-fertilized area was used to represent the relative yield (RY). The highest relative yield (HRY), the average relative yield (ARY) and the relative yield gap (G_RY) were obtained by using the statistical of high-yielding households, the effects of fertilization and soil factors on the relative yield gap were determined used the random forest model, and soil productivity level was divided according to the yield of non-fertilized area. The relationship between the relative yield gap of wheat and maize and nitrogen application rate under different soil productivity levels was quantified.

HRY of wheat in China was 3.83-6.75 t·hm^-2, ARY was 2.10-3.42 t·hm^-2, and G_RY was 1.73-3.33 t·hm^-2, G_RY accounting for 44.64%-49.06% of HRY. HRY, ARY and G_RY of wheat were north China>middle-lower Yangtze Plain>northwest China>southwest China. HRY of maize in China was 6.53-8.20 t·hm^-2, ARY was 3.37-4.12 t·hm^-2, and G_RY was 3.16-4.08 t·hm^-2, G_RY accounting for 44.78%-50.52% of HRY. HRY, ARY and G_RY of maize were northeast China>north China>southwest China>northwest China. Except for north China, HRY and G_RY of wheat and maize increased with time. Except in northwest China, the G_RY decreased with the increase of nitrogen application rate in low and medium soil productivity, and the decrease amplitude was more significant in low soil productivity level, while the decrease of G_RY with nitrogen application rate in high soil productivity was not significant. Regionally, the balance points of nitrogen fertilizer application were found in wheat and maize in North China, wheat in middle-lower Yangtze Plain, and maize in northeast China at low and medium soil productivity. Overall, the nitrogen application rate and soil organic matter were relatively important influencing factors of G_RY for wheat and maize at low and medium soil productivity. Potassium application had a significant impact on the G_RY in middle-lower Yangtze Plain and north China, while organic matter had a significant impact on the G_RY in the northwest and southwest China under high soil productivity.

N application and soil organic matter were important factors affecting the relative yield gap. The higher soil productivity level, the lower the effect of nitrogen fertilizer on reducing the relative yield gap. N fertilizer should be reduced appropriately in high productivity soil. In order to increase yield and avoid the waste of resource and environmental risks, it was suggested that the application rate of nitrogen fertilizer should not exceed its balance point. The recommended application rates of nitrogen fertilizer were 260.6 and 159.2 kg·hm^-2 for wheat and 262.5 and 246.0 kg·hm^-2 for maize at low and medium productivity levels in north China, respectively. In the middle-lower Yangtze Plain, 199.5 and 187.5 kg·hm^-2 were recommended for nitrogen application at low and medium productivity levels, respectively. In northeast China, the recommended amount of N fertilizer application was 259.5 and 228.0 kg·hm^-2, respectively. Under low and medium productivity levels in southwest and northwest China, N fertilizer should be appropriately increased. The potassium fertilizer reasonable application should be paid more attention at high soil productivity in north China and middle-lower Yangtze Plain. The improvement of soil organic matter should be as the main measures to achieve high and stable yields in southeast and southwest China.

China has rich wheat and maize stalk resources. It is necessary to clarify the spatial-temporal characteristics of the wheat and maize stalk resources, the corresponding nutrient resources and chemical fertilizer reduction potential through stalk incorporation, which can provide decision basis for promoting the utilization of stalk resources and reducing chemical fertilizer application.

Based on the soil long-term monitoring data of the Ministry of Agriculture and Rural Affairs in wheat and maize planting areas of China from 1988 to 2019, this study analyzed the amount of wheat and maize stalk and its nutrient resources and fertilizer reduction potential through stalk incorporation in different years in each region of China.

The annual average amount of wheat and maize stalk resources reached 1.62×10⁸ t and 4.23×10⁸ t, respectively in 2010s, which were increased by 0.16×10⁸ t and 2.04×10⁸ t compared with 1990s, respectively. The annual average amount of wheat and maize stalk NPK resources reached 278.19×10⁴ t and 901.08×10⁴ t, respectively, which were increased by 27.97×10⁴ t and 434.82×10⁴ t compared with 1990s, respectively. Both of them increased most in North China (NC). The annual average amount of wheat stalk resources and NPK resources decreased first and then increased with planting years, while maize increased. In the first stage (1990s to 2000s) and the second stage (2000s to 2010s), the annual variation rate (AVR) of wheat stalk resources were -42.47×10⁴ t·a^-1 and 205.10×10⁴ t·a^-1, and the AVR of nutrient resources were -0.26×10⁴ t·a^-1 of N, -0.03×10⁴ t·a^-1 of P, -0.44×10⁴ t·a^-1 of K and 1.27×10⁴ t·a^-1 of N, 0.14×10⁴ t·a^-1 of P, and 2.11×10⁴ t·a^-1 of K, respectively. In the first stage and the second stage, the AVR of maize stalk resources were 397.82×10⁴ t·a^-1 and 1 643.60×10⁴ t·a^-1, and the AVR of nutrient resources were 3.46×10⁴ t·a^-1 of N, 0.56×10⁴ t·a^-1 of P, 4.46×10⁴ t·a^-1 of K and 14.30×10⁴ t·a^-1 of N, 2.30×10⁴ t·a^-1 of P, and 18.41×10⁴ t·a^-1 of K, respectively. There were more than 80% of wheat stalk and its nutrient resources distributed in NC and Middle and lower reaches of Yangtze River (MLRYR), with the highest in NC (0.93×10⁸ t, 160.31×10⁴ t of NPK), and the lowest in Southwest China (SW) (0.09×10⁸ t, 16.05×10⁴ t of NPK). About 70% of maize stalk and its nutrient resources were distributed in Northeast China (NE) and NC, with the highest in NE (1.39×10⁸ t, 296.96×10⁴ t of NPK), and the lowest in MLRYR (0.21×10⁸ t, 44.40×10⁴ t of NPK). The annual average nutrient-release amount of wheat stalk incorporation per unit area were 21.1 kg·hm^-2 of N, 3.0 kg·hm^-2 of P and 62.3 kg·hm^-2 of K in China, with the highest in NC, and the lowest in SW. The annual average nutrient-release amount of maize stalk incorporation per unit area were 48.6 kg·hm^-2 of N, 10.6 kg·hm^-2 of P and 97.7 kg·hm^-2 of K in China, with the highest in Northwest China (NW), and the lowest in SW. The annual average nutrient-release amount of wheat and maize stalk incorporation per unit area increased continually during 1988-2019. The proportion of wheat stalk returning nutrients to the annual chemical fertilizer application were 9.13%-10.82%, 4.26%-6.43% and 88.02%-111.86%, respectively, and that of maize stalk were 22.99%-24.37%, 16.04%-28.67% and 150.29%-171.95%, respectively.

In general, using wheat and maize stalk properly could satisfy the basic potassium requirement for crop production and reduce the application of about 10%-20% nitrogen fertilizer and 5%-20% phosphorus fertilizer. Making full use of stalk resources was an important guarantee for reducing fertilizer application and increasing efficiency.