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Biochar amendment can reduce CH 4 emissions from paddy soils. However, little is known about how the soil microbial communities associated with paddy soil CH 4 emissions respond to biochar aging after biochar amendment. In this study, we examined the effects of biochar on CH 4 emissions, soil properties, and abundance/community composition of methanogens and methanotrophs in a double rice cropping system from 2012 to 2016. Straw-derived biochar was applied once in 2012 at 24 and 48 t ha−1. Biochar application decreased the annual CH 4 emissions by 20–51%. Biochar increased the abundances of both methanogens and methanotrophs, with a larger increase of methanotrophs than methanogens in the first year, mainly caused by the increases in soil dissolved organic carbon, NH 4 +-N, and porosity. Biochar suppressed the abundance of methanogens and had little effect on the methanotrophs in the following three years, probably due to the increased soil porosity. Eventually, the ratios of abundance of methanogens to methanotrophs decreased by 11–31% in each of the four years and were positively correlated to CH 4 emissions. Biochar addition increased the relative abundances of Methanocella and Methanospirillum and reduced those of Methanoregula and Methanosaeta for methanogens, while it increased the proportion of the basophilic methanotrophs Methylomicrobium and decreased that of Methylocaldum in the growing season 2014. Our results demonstrate that biochar aging greatly alters the responses of abundances and community compositions of soil methanogens and methanotrophs to biochar addition. The reduction of CH 4 emissions owing to biochar in the long run was probably mainly due to the lesser suppression of abundance and activity of methanotrophs compared with methanogens. • Biochar decreased paddy CH 4 emissions across four years. • Biochar significantly reduced methanogen:methanotroph ratios in four years. • Fresh biochar with nutrients stimulated methanotrophs and methanogens in year one. • Aged biochar suppressed methanogen growth in years 2–4 due to increased soil porosity. • Biochar reduced the acetoclastic methanogen abundances at tillering and ripening. [ABSTRACT FROM AUTHOR]

Poor water and nutrient retention are the major soil fertility limitations in the low productivity agricultural soils of Nepal. The addition of biochar to these soils is one way these hindrances can be overcome. In the present study, six different biochar doses (control, 5 t ha −1 , 10 t ha −1 , 15 t ha −1 , 25 t ha −1 and 40 t ha −1 ) were applied to a moderately acidic silty loam soil from Rasuwa, Nepal and the effects on soil physicochemical properties and maize and mustard yield over three years (i.e., six cropping seasons), were investigated. Biochar addition did not show significant effects on maize and mustard grain yield in the first year, however significant positive effects (p   <   0.01) were observed during the second and third years. During the second year, maize grain yield significantly increased by 50%, 47% and 93% and mustard grain yield by 96%, 128% and 134% at 15 t ha −1 , 25 t ha −1 and 40 t ha −1 of biochar respectively. A similar significant increase in yield of both crops was observed in the third year. Yields for both maize and mustard correlated significantly (p   <   0.001 ) with plant available P, K + , pH, total OC%, CEC, base saturation, and increased as a function of biochar addition. On the basis of the measured crop yields for the various biochar doses, a cost-benefit analysis was carried out, and gross margin was calculated to optimize biochar dose for local farming practice. Total costs included financial cost (farm input, labor and biochar production cost), health cost and methane emission cost during biochar production. Health costs were a minor factor (<2% of total biochar preparation cost), whereas methane emission costs were significant (up to 30% of biochar cost, depending on the C price). Total income comprised sale of crops and carbon sequestration credits. The cost-benefit analysis showed that the optimal biochar application dose was 15 t ha −1 for all C price scenarios, increasing gross margin by 21% and 53%, respectively, for 0 and 42 US$ per ton CO 2 price scenarios. In the current situation, only the 0 US$ price scenario is realistic for rural farmers in Nepal, but this still gives benefits of biochar amendment, which are capped at a 15 t ha −1 biochar addition. [ABSTRACT FROM AUTHOR]

• Straw biochar increased the abundances of paddy soil bacteria and fungi 3–4 years after amendment. • Biochar amendment decreased soil bacteria/fungi ratios. • Biochar improved soil bacterial composition to favor TOC accumulation. • Biochar improved soil fungal composition to favor plant growth and TOC degradation. • Biochar reduced fungal abundances of plant pathogen in paddy soil. The effects of biochar application on soil microbial communities and functional characteristics and their correlations with soil fertility properties were explored in a double rice cropping system three to four years after a single biochar amendment. Three treatments including a control, a low (24 t ha−1), and a high (48 t ha−1) application rate of straw-derived biochar were constructed. Biochar amendment significantly increased the abundance of bacteria and fungi by up to 102 % and 178 %, respectively, which might be probably caused by the increases in soil total organic carbon (TOC), total nitrogen, and rice biomass as compared with the control. However, the abundance of archaea was only slightly elevated after biochar amendments. Bacteria/fungi ratios were significantly decreased by up to 61.4 % in the biochar treatments, probably because fungi were the dominant decomposers of increased recalcitrant carbon from biochar and rice biomass. Biochar stimulated the relative abundance of Acidobacteria, which favours soil organic carbon accumulation. Biochar increased the relative abundances of Mortierella and Westerdykella , which are more beneficial to plant growth and TOC degradation. Furthermore, potential phytopathogens of Athelia and Penicillium were decreased with biochar amendment. The results demonstrate that biochar application should be sustained as an effective measure for improving the microbial characteristics of paddy field by ameliorating its soil properties. [ABSTRACT FROM AUTHOR]

• Phosphorus (P) input and crop type cause differences in P loss via runoff. • Modified biochar reduced the runoff P concentration and loss load in all cropping systems. • Solid organic fertilizer decreased particulate P and colloidal P losses. • Biogas slurry increased the P loss in the runoff of the rice-wheat rotation system. Agricultural cropping systems involve the application of high fertilizer rates, which lead to phosphorus (P) losses via surface runoff to the aquatic environment, thereby resulting in severe eutrophication. Here, to evaluate the nutrient loss, we established three monitoring stations in Zhejiang Province, China, in each of which the crop types were double cropping rice, rice–wheat rotation, and vegetable cultivation. Field experiments were conducted with four treatments and three replications over a whole planting year. The four treatments at the same P application rate were: 1) no fertilization (control; CK), 2) chemical fertilization (CF), 3) substitution of 30 % chemical P in CF by organic P in solid sheep manure (OF) or in liquid biogas slurry (BS), and 4) substitution of 30 % chemical P in CF by organic P fertilizers (solid sheep manure or liquid biogas slurry) +1.5 t ha−1 polyacrylamide modified BC (PSB). The runoff losses of various P forms, that is, the total P, total dissolved P, particulate P, and colloidal P losses were analyzed. The total P concentration varied greatly with the crop growth stages. Compared with CF treatment, PSB treatment significantly reduced the concentration of P fractions in the runoff in all three cropping systems, while the OF treatment significantly reduced the particulate P and colloidal P concentrations by coating colloidal particles with organic carbon. However, BS treatment increased particulate P and colloidal P concentrations. Compared with CF treatment, PSB treatment reduced total P loss by 41.1 %, 29.7 %, and 37.8 % in the double cropping rice, rice–wheat, and vegetable systems, respectively. The PSB and OF treatments significantly reduced the particulate P and colloidal P losses, while BS treatment increased the loss of various P forms. Compared with the CF treatment, the PSB treatment significantly increased the available P contents by 12.7–85.4 mg kg-1, and decreased the colloidal P contents by 26.7 %–51.4 % in the soils examined. The results of the redundancy analysis confirmed that P loss via runoff was mainly caused by differences in the P inputs and crop types. The results of this study highlight the important role of PSB in reducing P loss via runoff from agricultural cropping systems. [ABSTRACT FROM AUTHOR]

• SOC and grain yield was measured with biochar and straw addition in maize-rice vs rice-rice system. • Straw addition has no effect on SOC accumulation for both cropping systems over a 4-year periods. • Biochar significantly improved more SOC in rice-maize system than rice-rice system. • Higher SOC under rice-maize system with biochar addition were linked increased soil aggregation. • Both biochar and straw addition led to higher late rice yield. Crop rotation of flooded rice with an upland crop like maize on previous continuous paddy soils is an emerging cropping system in South China. A four-year experiment was conducted at a research area in Hunan province, having a long history of double paddy-rice cropping throughout the year. Maize was introduced as an upland crop in rotation with paddy rice, thus providing two parallel cropping systems i.e., the previous flooded rice-rice (R–R) and the new maize-rice rotation (M–R) systems. We used three treatments in both cropping systems; namely, farmer's practice (without external C input) as a control, straw addition, and biochar addition. The straw and biochar were added to soil in late rice season on an equal C input basis (3000 kg C ha−1yr-1). In the R–R plots, rice straw was added while in M–R plots maize straw was added during late rice field preparation. Our results show that there were no changes in soil organic carbon (SOC) concentration when R–R was replaced by M–R rotation. Straw addition had no effect on SOC but improved late rice yield. Biochar addition significantly increased SOC and late rice yield in both cropping systems. Moreover, biochar addition resulted in a more significant accumulation of SOC (9% higher) in M–R than R–R. Among soil aggregates, only the 0.25–2 mm fraction had a significantly higher SOC concentration with biochar relative to the control and straw return; and this increase was significantly more pronounced under M–R (28% higher) than R–R cropping system. In general, a significantly higher proportion of small macro-aggregates (0.25–2 mm) were found under M–R, while smaller aggregates (0.05–0.25 and <0.05 mm) were more abundant under R–R. δ13C analyses from M–R plots reveal that the quantity of maize-derived C was marginally higher in the straw treatment than control, but this was not reflected in any significant impact on total SOC of the bulk soil or of aggregates. In conclusion, no detectable changes in SOC were observed over four years experimental period following shift from R-R to M-R cropping system. M-R system with biochar addition led to higher productivity and environmental benefits. [ABSTRACT FROM AUTHOR]