Biochar is increasingly used in crop production as a fertilizer; however, its effects on nutrient cycling and stoichiometry in rice paddy soil–plant systems are unclear. We tested for effects of contrasting rates of biochar on soil and rice plant organ carbon (C), nitrogen (N), and phosphorus (P) concentrations and stoichiometry and soil physicochemical properties in early and late paddies. Overall, biochar reduced soil bulk density by an average of 7.4%, while application at 10, 20, and 40 t ha−1 increased soil C and N concentrations in early paddies by 31.6, 41.3, and 104.2%, respectively, and by 8.0, 5.0, and 21.8%, respectively; in late paddies, there were increases of 23.0, 94.1, and 117.0%, respectively, and 6.7, 15.4, and 18.0%, respectively (P < 0.05). Following biochar application at 10, 20, and 40 t ha−1, soil concentration of P decreased in early paddies by 10.9, 19.0, and 13.9%, respectively, and increased in late paddies by 4.3, 16.4, and 20.1%, respectively. Biochar increased ratios of soil C:N and C:P in early and late paddies (P < 0.05), and there was no effect on concentration and stoichiometry of soil available nutrients. Biochar reduced rice plant organ concentration of N and P in early rice and increased leaf N:P ratios. Despite the biochar application improved nutrient status in plant–soil system, we did not observe a significant increase in yield (P > 0.05). According to the N:P value of leaves between treatments, it was found that biochar alleviated the current situation of N limitation in paddy fields during the mature period and transformed the N limitation of early rice into a joint limitation of N and P. These results show that the addition of biochar to subtropical paddy soils leads to a short-term reduction in soil bulk density and increases in soil C and N concentrations and soil fertility. Thus, biochar applied at optimal rates is likely to improve the sustainability of subtropical paddy rice production.

N-enriched biochar can increase the accumulation of carbon (C), nitrogen (N), phosphorus (P), and biomass in rice plants. On the other hand, the biomass and C, N, and P contents of plant organs are important indicators to reflect plant C, N, and P storages. We established control, 4 t ha−1, and 8 t ha−1 N-enriched biochar treatment plots in a subtropical paddy field in China to investigate the effect of these treatments on C, N, and P storages, ecological stoichiometry in various rice plant organs, and their relationships with edaphic factors. The application of N-enriched biochar increased the biomass and storages of C, N, and P in rice roots, stems, leaves, and grains, mainly at 4 t ha−1. The application of N-enriched biochar decreased the C/N and C/P ratios of rice organs, but increased their N/P ratio. Changes in C/N were mainly due to the changes in storage, while N/P was positively correlated with N storage of stems, leaves, and grains and negatively correlated with P storage in roots. Pearson’s correlation analysis revealed that pH was negatively correlated, and soil N content was positively correlated with P storage in various organs of rice. In addition, soil P content and chlorophyll were positively correlated with N storage. In conclusion, we found that the application of N-enriched biochar improved plant N and P storage and stoichiometrical relations among rice organs.

Application of biochar to rice has shown to elicit positive environmental and agricultural impacts due to its physicochemical properties. However, the relationship between greenhouse gas (GHG) emissions, rice yield, and soil nutrient status under biochar amendment remains unclear. In this study, rice yield and methane (CH4) and nitrous oxide (N2O) emissions were quantified in response to biochar application rate (0, 10, 20, and 40 t ha−1) to early and late subtropical rice cropping systems. We found that application of 10 t of biochar ha−1 to early rice reduced average CH4 emission fluxes, while all biochar application rates diminished average emissions in late rice paddy. Total global warming potential (GWP) and GHG intensity (GHGI) were inherently greater in late rice than early rice cropping. In early rice, GWP and GHGI were found to be similar between soil control, 10 and 20 t of biochar ha−1 treatments, although the largest occurred in the 40 t of biochar ha−1 treatment, whereas in late rice cropping, they were not affected by biochar application rates. Compared to the nil-biochar application, biochar application at varied rates did not affect rice yield. However, compared to 10 t biochar ha−1, increasing biochar application rate to 40 t ha−1 significantly decreased total rice yield (sum of early and late cropping). Generally, application of biochar increased soil salinity and total Fe and Fe2+ content while reducing soil bulk density. Temporal effects of biochar application were noted on CH4 emission flux, soil temperature, and soil Fe2+ and Fe3+ in early rice; and soil temperature, salinity, NH4+-N, NO3−-N, and soil Fe2+ and Fe3+ in late rice. This study confirms that the application of biochar at the lower rate of 10 t ha−1 is optimal for maintaining rice yield while reducing GHG emissions. Moreover, the study demonstrates the potential benefit of biochar in sustainable subtropical rice production.

Soil degradation is characterized by loss of soil organic matter, decline in fertility, imbalance in elemental content, deterioration of soil structure, and overall a deterioration of soil environment. According to the classification method of Pieri et al. (1992), the soil is classified into different degradation classes by calculating the soil structural stability index (St) of each sample point. We aimed to investigate changes in the contents, storages and stoichiometry of soil carbon (C), nitrogen (N), and phosphorus (P) together with changes in soil physical traits along a soil degradation gradient in jasmine croplands in Fuzhou area (China). The content and storage of soil C and N decreased with increasing intensity of land degradation. Soil organic C content was 15.4%, 32.3%, and 38.8% lower, respectively, in the low, medium, and high degree of degradation soils, than in the nondegraded soils. The soil C:N ratio was 18.5% higher in soils in the middle degree of degradation than in the nondegraded soils. Compared with nondegraded soils, the bulk density of the degraded soils increased and water content decreased. The decrease of soil pH coupled with salinity (conductivity) and the loss of aggregate stability are the main traits that distinguish degraded from nondegraded soils. We also detected a general N and P deficiency that is aggravated by the degradation process. Unreasonable management easily leads to degradation associated with a loss of organic C and total soil nutrients, thus impairing even more a general N and P deficiency in this area. Therefore, higher inputs of organic fertilizer should be added to alleviate the lack of organic matter, and appropriate burial should be conducted to reduce nutrient loss. Moreover, a rise of N and P fertilizer application is also advisable.

The effects of straw alone or combined with industrial and agricultural wastes as fertilizers on greenhouse gas (GHG) emissions are still poorly known in cropland areas. Here, we studied the effects of 3.5 Mg ha−1 straw and 3.5 Mg ha−1 straw combined with 8 Mg ha−1 of diverse wastes on GHG emission in a subtropical Jasminum sambac plantation in southeastern China. There were five treatments in a completely randomized block design: control, straw only, straw + biochar, straw + steel slag, and straw + gypsum slag. Emissions of carbon dioxide were generally higher in the treatments with waste than in the control or straw-only treatments, whereas the contrary pattern was observed in CH4 and N2O emission rates. Moreover, the total global warming potentials (GWPs) were no significantly higher in most of the amended treatments as compared to the control and straw-only treatments. In relation to the treatment with only straw, GWPs were 9.4% lower when steel slag was used. This finding could be a consequence of Fe amount added by steel slag, which would limit and inhibit the emissions of GHGs and their transport from soil to atmosphere. Our results showed that the application of slags did not increase the emission of GHGs and that the combination of straw with steel slag or biochar could be more effective than straw alone for controlling GHGs emission and improve soil C and nutrient provision.

Suitable fertilization is crucial for the sustainability of rice production and for the potential mitigation of global warming. The effects of fertilization on porewater nutrients and greenhouse-gas fluxes in cropland, however, remain poorly known. We studied the effects of no fertilization (control), standard fertilization and double fertilization on the concentrations of porewater nutrients, greenhouse-gas fluxes and emissions, and rice yield in a subtropical paddy in southeastern China. Double fertilization increased dissolved NH4+ in porewater. Mean CO2 and CH4 emissions were 13.5% and 7.4%, and 20.4% and 39.5% higher for the standard and double fertilizations, respectively, than the control. N2O depositions in soils were 61% and 101% higher for the standard and double fertilizations, respectively, than the control. The total global warming potentials (GWPs) for all emissions were 14.1% and 10.8% higher for the standard and double fertilizations, respectively than the control, with increasing contribution of CH4 with fertilization and a CO2 contribution > 85%. The total GWPs per unit yield were significantly higher for the standard and double fertilizations than the control by 7.3% and 10.9%, respectively. The two levels of fertilization did not significantly increase rice yield. Prior long-term fertilization in the paddy (about 20 years with annual doses of 95 kg N ha−1, 70 kg P2O5 ha−1 and 70 kg K2O ha−1) might have prevented these fertilizations from increasing the yield. However, fertilizations increased greenhouse-gas emissions. This situation is common in paddy fields in subtropical China, suggesting a saturation of soil nutrients and the necessity to review current fertilization management. These areas likely suffer from unnecessary nutrient leaching and excessive greenhouse-gas emissions. These results provide a scientific basis for continued research to identify an easy and optimal fertilization management solution.

Rice is the main food for most of the human population, so sustainable rice production is very important for food security. The fertility of the soil in paddy fields is the key factor controlling rice growth and production. Steel slag amendment is becoming an effective method to increase the soil fertility, stabilize rice production and reduce greenhouse-gas emissions in Asiatic paddy fields (i.e. Korea, Japan, Bangladesh and China). We studied the relationships of steel slag amendment with plant–soil nutrient allocation, stoichiometry and rice yield in a paddy field in subtropical China. Amendment was associated with higher soil N and P availability, lower available-N:available-P ratio and higher available Ca and Si concentrations. Increases in P, Ca and Mg availability were correlated with high yields. High yields under steel slag amendment were also associated with high foliar and stem N and P concentrations and lower N:P ratios and with high shoot/root N and P concentration ratios, traits that are typically associated with productive ecosystems able to support species with high growth rates. The positive correlation between steel slag application and yield was partially due to an indirect effect (35% of the total effect) of enhancement of soil Ca, Si and P availability, which were positively correlated with yield. Steel slag amendment in this paddy field increased plant growth and yield by enhancing nutrient availability, altering soil and plant stoichiometry and shifting stem:root nutrient allocation.

Reducing the emissions of greenhouse gases (GHG) from paddy fields is crucial both for the sustainability of rice production and mitigation of global climatic warming. The effects of applying industrial and agricultural wastes as fertilizer on the reduction of GHG emissions in cropland areas, however, remain poorly known. We studied the effects of the application of 8 Mg ha−1 of diverse wastes on GHG emission and rice yield in a subtropical paddy in southeastern China. Plots fertilized with steel slag, biochar, shell slag, gypsum slag and silicate and calcium fertilizer had lower total global-warming potentials (GWP, including CO2, CH4 and N2O emissions) per unit area than control plots without waste application despite non-significant differences among these treatments. Structural equation models showed that the effects of these fertilization treatments on gas emissions were partially due to their effects on soil variables, such as soil water content or soil salinity. Steel slag, biochar and shell slag increased rice yield by 7.1%, 15.5% and 6.5%, respectively. The biochar amendment had a 40% lower GWP by Mg−1 yield production, relative to the control. These results thus encourage further studies of the suitability of the use waste materials as fertilizers in other different types of paddy field as a way to mitigate GHG emissions and increase crop yield.