Biochar modification has received significant research attention due to its attractive and fruitful rewards in improving biochar performances. However, the determinants of modified biochars’ stability and the ability of aged modified biochars to remove heavy metals have not been comprehensively evaluated. Therefore, three commonly used functional groups of modified biochars (5% and 10% H₂O₂-modified (BCH5 and BCH20); 25% and 65% HNO₃-modified (BCA25 and BCA65); and amino-modified (BCN), were prepared in this study to explore their stability and the Cd(II) removal performance of aged functional groups modified biochars was studied. The results showed that the O/C ratio is not sensitive enough to predict the stability of functional groups modified biochars, which was commonly used to evaluate pristine biochar (BC0); instead, -COOH content is crucial for modified biochar stability (r = −0.99, r = −0.91, p < 0.05). BCA65 displayed the highest less prone to oxidation property, which indicated that the high surface charges and uniform surface determined the less prone to oxidation ability of the functionally modified biochars. The order of the stability of functionally modified biochars was HNO₃-modified > H₂O₂-modified > amino-modified. After oxidation, the surface charges and pores were significantly reduced, and the Cd²⁺ adsorption mechanism of modified biochar changed from multilayer adsorption to monolayer adsorption, which led to a reduction in overall Cd(II) removal. The maximum adsorption capacities of BCA65 were reduced from 18.15 mg·g⁻¹ to 4.86 mg·g⁻¹ after oxidation, particularly. In the design and preparation of modified biochar, the long-term stability of the structure and function of modified biochar and its sustainable application in the environment should be considered while improving the heavy metal removal performance of biochars.

With the frequent occurrence of global droughts, modified biochar has demonstrated the potential to be an efficient soil amendment, which could affect carbon and nitrogen sequestration in arid soil. Therefore, this study investigated the co-application of pristine biochar (BC), Fe-modified biochar (FB) and H₂O₂-modified biochar (HB) with green manure during a 70-day laboratory incubation under drought conditions and normal moisture conditions. The emissions were quantified using gas chromatography, while microbial necromass carbon and nitrogen were measured by quantifying the amino sugar content by gas chromatography–mass spectrometry, and other soil carbon and nitrogen fractions were determined through chemical analysis. The results revealed that under drought conditions, compared to BC co-application with green manure, the total carbon loss of FB and HB co-application with green manure was reduced from 24.38% to 13.14% and 14.27%, respectively, and the total nitrogen loss was also reduced from 14.61% to 7.23% and 7.27%, respectively. This reduction occurred because FB and HB protected soil organic matter through iron oxide binding and micropore adsorption, thereby increasing the content of soil total humus acid (>5%) and microbial necromass nitrogen (>16%). In addition, under normal moisture conditions, BC, FB and HB co-application with green manure enhanced microbial activity and promoted the formation of stable total humus acid, thereby enhancing carbon and nitrogen sequestration. In conclusion, this study provides crucial theoretical support for the optimization of the green manure return via modified biochar co-application in arid environments.

The impact of biochar on N conversion in drought-affected soil with green manure application has not been systematically investigated. Therefore, this study examined the effects of biochar produced at 350 °C, 550 °C, and 750 °C (BC350, BC550, and BC750) on green manure (Medicago sativa L.) decomposition and N turnover in soil (Hapli-Udic Ferralosol) during a 60-day laboratory incubation experiment under different moisture conditions (45% or 65% soil water-holding capacity (WHC)). Due to low-temperature biochar (BC350) possessing hydrophilicity (thereby enhancing water retention) due to its surface oxygen-containing functional groups, while high-temperature biochar (BC750) relies on the aromatic conjugated π-electron system for electron transfer, both BC350 and BC750 promoted green manure decomposition and mineralization. Compared with green manure alone, co-application of BC350 and BC750 with green manure increased soil NH₄⁺–N content by 5.58% and 38.37%, respectively. However, a significant total N loss (>11%) occurred under drought conditions. Partial least squares path modeling was used to elucidate the key driving pathways related to C and N sequestration in drought-affected soil with green manure and biochar co-application. The results indicate that the enhanced soil N loss could be attributed to both the biochar-induced rise in soil pH (>8%) and the drought-driven suppression of stable organic matter (e.g., >20% reduction in humus acid) and macroaggregate formation. This study demonstrates that although biochar addition can promote the decomposition of green manure to release available N, it may also exacerbate total soil N loss. Only under normal moisture conditions can the N released from green manure be converted into humus-associated N.