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Potassium (K) deficiency is commonly observed during crops grown in the red soil (which is a typical Plinthosol based on the IUSS guidelines) of China, but few studies have examined soil aggregate-associated K. In a long-term field experiment (initiated in 1986), the following treatments were applied: no fertilizer (CK), nitrogen and phosphorus fertilizers (NP), NP and K fertilizers (NPK) and a combination of manure and NPK (NPKM). After 30 years of fertilization, the nonexchangeable K (Nonex-K), and exchangeable K (Ex-K) contents or stocks in most aggregates varied among different treatments. Compared with NP treatment, the Nonex-K contents in the aggregate fractions of >2-, 1–2-, 0.5–1-, 0.25–0.5- and 0.053–0.25-mm under NPKM treatment were increased by 40.57%, 40.78%, 42.71%, 40.82% and 55.43%, respectively. The Nonex-K contents in 0.5–1-, 0.25–0.5-, 0.053–0.25- and 2-, 1–2-, 0.5–1- and 0.053–0.25-mm aggregates of NPKM treatment were significantly increased by 74.36%, 123.63%, 44.88% and 37.47%, respectively, compared with those of the NPK treatment. Furthermore, a random forest model showed that K stocks in the >2-, 1–2- and 0.5–1-mm aggregates were the main factors affecting the uptake of K by maize. The relationships between the uptake of K by maize and K stocks in the >2-, 1–2- and 0.5–1-mm aggregates could be fitted by linear equations. Therefore, the long-term combination of chemical fertilizers with manure improved K contents and stocks for most aggregate sizes in red soil, especially Ex-K. We found that the turnover rate of K in the >2-mm aggregates was faster than in the other aggregates (1–2- and 0.5–1-mm) through slopes of linear regressions, allowing the soil to meet the requirements for crop K uptake.

There is limited information on carbon sequestration efficiency (CSE) of soil aggregates in upland and paddy soils under long-term fertilization regimes. In a red soil region of southern China, an upland soil experiment started in 1986 and a paddy soil experiment commenced in 1981. These experiments were conducted using different fertilization treatments. After 30 years, soil organic carbon (SOC) content and stock of different aggregate components were analyzed. The results showed that the SOC contents and stocks in upland soil were lower than in paddy soil. In both upland and paddy soils, the SOC contents and stocks of all aggregate components in NPKM (combined treatment with chemical nitrogen (N), phosphorus (P), potassium (K) fertilizers and manure) were the highest among all treatments. Compared with CK (no fertilizer), SOC content of all aggregate components in NPKM was increased by 13.21−63.11% and 19.13−73.33% in upland and paddy soils, respectively. Meanwhile, the change rates in SOC stock of all aggregate components in upland soil were lower than in paddy soil, although the change rate of SOC stock of all aggregate components in NPKM was higher than in other treatments. Furthermore, a linear equation could fit the relationships between carbon (C) input and change rate of SOC stock (P

Soil enzyme activity and microorganism community can be changed through different long-term fertilization patterns. However, the effect of different fertilization practices on soil microorganisms might differ among crop systems. The objective of the study was to reveal the change of soil enzyme activity and soil microorganism community in different fertilizations both in upland and paddy soils. Therefore, based on long-term fertilization experiments in upland soil started in 1986 and adjacent paddy soil experiment commenced in 1981, with both consisting of 4 treatments: Control (no fertilization), N (only nitrogen fertilizer), NPK (nitrogen, phosphate and potassium fertilizers) and NPKM (nitrogen, phosphate and potassium fertilizers plus organic manure), grain yield, soil fertility, activities of soil urease, catalase, acid phosphatase, microorganism community (the number of bacteria, fungus and actinomycete) were analyzed. The result showed that: the highest grain yield was attained under the application of chemical fertilizers plus manure, as compared with Control, NPKM significantly increased the grain yield by 908.63% in corn and 118.80% in rice (p

Background Rice canopy changes are associated with changes in the red light (R), green light (G), and blue light (B) value parameters of digital images. To rapidly diagnose the responses of rice to nitrogen (N) fertilizer application and planting density, a simple model based on digital images was developed for predicting and evaluating rice yield. Results N application rate and planting density had significant effects on rice yield. Rice yield first increased and then decreased with increasing of N rates, while the rice yield always increased significantly with increasing planting density. The normalized redness intensity (NRI), normalized greenness intensity (NGI), and normalized blueness intensity (NBI) values of the rice canopy varied among stages; however, they were primarily affected by N fertilizer rates, while planting density had no significant effects. Furthermore, the significant relationships of grain yield with NRI and NBI at the late filling stage could be fitted by quadratic equations, but there was no significant relationship observed between grain yield and NGI across all stages. In addition, a field validation experiment showed that the predicted yield based on the fitted quadratic equations was consistent with the measured yield. Conclusion The NRI, NGI, and NBI values of rice canopy were mainly affected by N fertilizer rates, while the planting density had no significant effect. The significant relationships between grain yield with NRI and NBI at the late filling stage could be fitted by quadratic equations. Therefore, the canopy NRI and NBI at the late filling stage as measured by digital photography could be used to predict grain yield in southern China. Electronic supplementary material The online version of this article (10.1186/s13007-019-0416-x) contains supplementary material, which is available to authorized users.

Soil pH adjusted by the application of lime can improve soil potassium (K) availability to meet crop K uptake in acidic red soil. However, the response of aggregated K to pH changes is poorly understood. Objectives of this study was to quantify the relationships between aggregate K content and pH levels in soil with different fertilization patterns from a long-term field experiment Then, an incubation experiment of pH adjustment was conducted over 90 days at different pH levels where HCl and Ca(OH)2 were added to nitrogen and phosphorus fertilizers (NP); NP and K fertilizers (NPK); and NPK with manure (NPKM) soils. Our results showed that double linear equations can fit the relationships between aggregate exchangeable K (EK) with pH levels. However, the slopes and other parameters of fitted equations varied among different fertilization soils. Linear equations indicated that increasing pH value could improve non-exchangeable K (NEK) contents of soil aggregates, especially in NP soil with lower initial pH. However, the proportions of EK and NEK stocks in soil aggregates were significantly changed by soil pH adjustment only in NPKM soil. Additionally, redundancy analysis and partial least squares path mode also suggested that soil pH only affected the NEK contents in soil aggregates, although fertilization had direct effects on the EK and NEK contents in soil aggregates. Therefore, this study demonstrated that the aggregate K distribution of adjusting pH varied among soils with different fertilization regimes, then, improved soil pH could maintain high EK and NEK content of soil aggregates in red soil.

Soil organic carbon (SOC) is one of the main carbon reservoirs in the terrestrial ecosystem. It is important to study SOC dynamics and effects of organic carbon amendments in paddy fields because of their vest expansion in south China. A study was carried out to evaluate the relationship between the SOC content and organic carbon input under various organic amendments at a long-term fertilization experiment that was established on a red soil under a double rice cropping system in 1981. The treatments included non-fertilization (CK), nitrogen-phosphorus-potassium fertilization in early rice only (NPK), green manure (Astragalus sinicus L.) in early rice only (OM1), high rate of green manure in early rice only (OM2), combined green manure in early rice and farmyard manure in late rice (OM3), combined green manure in early rice, farmyard manure in late rice and rice straw mulching in winter (OM4), combined green manure in early rice and rice straw mulching in winter (OM5). Our data showed that the SOC content was the highest under OM3 and OM4, followed by OM1, OM2 and OM5, then NPK fertilization, and the lowest under non-fertilization. However, our analyses in SOC stock indicated a significant difference between OM3 (33.9 t ha−1) and OM4 (31.8 t ha−1), but no difference between NPK fertilization (27 t ha−1) and non-fertilization (28.1 t ha−1). There was a significant linear increase in SOC over time for all treatments, and the slop of linear equation was greater in organic manure treatments (0.276–0.344 g kg−1 yr−1) than in chemical fertilizer (0.216 g kg−1 yr−1) and no fertilizer (0.127 g kg−1 yr−1).

Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.