Long-term manure application increased soil organic carbon and nitrogen mineralization through accumulation of unprotected and physically protected carbon fractions

Soil organic carbon(SOC)and nitrogen(N)mineralization are important biogeochemical processes associated with soil fertility.These processes are influenced by physically,chemically,and biologically stabilized SOC fractions,the mechanisms of which are not well known.The present study was conducted to evaluate the combined effect of manure and mineral fertilizers on the contents of SOC fractions to promote the mineralization of SOC and N.Treatments included:i)no fertilizer control(CK);ii)a combination of mineral N,phosphorus,and potassium fertilizers(NPK);iii)manure alone(M);iv)manure combined with NPK(MNPK);and v)a high dose of manure combined with NPK(hMNPK).The combined uses of manure and mineral fertilizers(MNPK and hMNPK)enhanced the accumulation of the unprotected coarse particulate organic carbon(C)fraction(cPOC)by 44%-72%compared to CK.Manure applications(M,MNPK and hMNPK)enhanced physically microaggregate-protected organic C(μagg),physicochemically protected organic C within the microaggregate-derived silt(μsilt)fraction(H-μsilt),and physicobiochemically protected organic C within theμsilt fraction(NH-μsilt)by 30%-56%,62%-150%,and 27%-51%,respectively.In contrast,all chemically and biochemically protected SOC fractions showed a minor response to manure application.Accumulation of cPOC,μagg,H-μsilt,and physicochemically protected organic C within the microaggregate-derived clay fraction(H-μclay)significantly contributed to the mineralization of SOC and N,resulting in a significant increase in rice grain yield with long-term manure application.In summary,long-term combined use of manure and mineral fertilizers improved SOC accumulation in unprotected and physically protected fractions,which enhanced SOC and N mineralization and benefited soil productivity in a rice-wheat cropping system.

Biochar reduces uptake and accumulation of polycyclic aromatic hydrocarbons(PAHs) in winter wheat on a PAH-contaminated soil

For years, biochar has been successfully used for the remediation of polycyclic aromatic hydrocarbons(PAHs) in contaminated soils, not only for improving their removal from soil but also for reducing their uptake by crops. However, the underlying mechanism of biochar application reducing PAH uptake and accumulation in winter wheat remains unclear. Pot trials were conducted on a PAH-contaminated soil amended with bamboo biochar, coconut shell biochar,and maize straw biochar(MSB) for an entire growth period of winter wheat. Compared with no biochar control(CK), application of the three types of biochar significantly(P < 0.01) reduced grain PAH concentration, total equivalent concentration(TEC), and incremental lifetime cancer risk(ILCR), indicating that biochar application, especially MSB, reduced the risk of exposure to PAHs in wheat grain. Furthermore, all three types of biochar significantly(P < 0.05)reduced PAH uptake and accumulation in wheat roots and stems, probably because biochar application enhanced the degradation of PAHs in the rhizosphere soil. Compared with CK, application of the three types of biochar significantly(P < 0.05) reduced the concentration of PAHs in the rhizosphere soil by15.9%–33.7%. It was found that the degradation rate of high-molecular-weight(HMW) PAHs(5-and 6-ring PAHs) was significantly(P < 0.05) higher than that of low-molecular-weight(LMW) PAHs(2–4-ring PAHs) regardless of the type of biochar used. Additionally, all three types of biochar significantly increased the relative abundance of the dominant bacterial phyla and genera in soil. Redundancy and correlation analyses also showed that there was a strong correlation between the removal rate of PAHs and dominant bacteria in the rhizosphere soil. This study indicated that biochar effectively reduced the health risk from dietary exposure to PAHs in wheat grains by increasing the abundance of bacteria related to PAH degradation, promoting the biodegradation of PAHs in the rhizosphere soil, and consequently reducing PAH uptake by wheat.