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Abstract. Iron oxide minerals play an important role in stabilizing organic carbon (OC) and regulating the biogeochemical cycles of OC on the earth surface. To predict the fate of OC, it is essential to understand the amount, spatial variability, and characteristics of Fe-bound OC in natural soils. In this study, we investigated the concentrations and characteristics of Fe-bound OC in soils collected from 14 forests in the United States and determined the impact of ecogeographical variables and soil physicochemical properties on the association of OC and Fe minerals. On average, Fe-bound OC contributed 37.8 % of total OC (TOC) in forest soils. Atomic ratios of OC : Fe ranged from 0.56 to 17.7, with values of 1–10 for most samples, and the ratios indicate the importance of both sorptive and incorporative interactions. The fraction of Fe-bound OC in TOC (fFe-OC) was not related to the concentration of reactive Fe, which suggests that the importance of association with Fe in OC accumulation was not governed by the concentration of reactive Fe. Concentrations of Fe-bound OC and fFe-OC increased with latitude and reached peak values at a site with a mean annual temperature of 6.6 °C. Attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) analyses revealed that Fe-bound OC was less aliphatic than non-Fe-bound OC. Fe-bound OC also was more enriched in 13C compared to the non-Fe-bound OC, but C ∕ N ratios did not differ substantially. In summary, 13C-enriched OC with less aliphatic carbon and more carboxylic carbon was associated with Fe minerals in the soils, with values of fFe-OC being controlled by both sorptive and incorporative associations between Fe and OC. Overall, this study demonstrates that Fe oxides play an important role in regulating the biogeochemical cycles of C in forest soils and uncovers the governing factors for the spatial variability and characteristics of Fe-bound OC.

Biochar and inorganic fertilizer when co‐applied have been reported to increase crop yield and enhance soil fertility. However, studies on this complementary effect on soil properties and noodle rice performance in China are still scanty. To investigate the effects of biochar application coupled with inorganic fertilizers on soil sustainability and yield and yield attributes of noodle rice, outdoor pot experiments were conducted in the early and late growing seasons in 2018. The treatment combinations were T1 (B0 t/ha + N270 kg/ha), T2 (B20 t/ha + N270 kg/ha), T3 (B40 t/ha + N270 kg/ha), T4 (B60 t/ha + N270 kg/ha), T5 (B0 t/ha + N360 kg/ha), T6 (B20 t/ha + N360 kg/ha), T7 (B40 t/ha + N360 kg/ha), and T8 (B60 t/ha + N360 kg/ha). The results compiled across the seasons showed an increase in Pn (net photosynthetic rate), grain yield, N uptake, gel consistency, amylose content (AC), and protein content in biochar‐treated pots as compared to T1. Average increases of 63.24, 63.66, 14.85, 58.0, 59.0, 22.39, and 2.9% were observed in soil porosity, moisture content, pH, organic carbon, total nitrogen, available phosphorus, and available potassium in T4 over T1 across the seasons, respectively. Root morphological characteristics such as total root length, surface area, volume, and average root diameter were significantly improved in T3, T4, T7, and T8. Starch‐related enzymes such as starch branching enzyme (SBE), starch debranching enzyme (DBE), and soluble starch synthase (SSS) were not affected significantly; however, granule‐bound starch synthase (GBSS), ADP‐glucose pyrophosphorylase (ADPG), and starch synthesis (SS) enzyme showed higher activity in 40 and 60 t B/ha across N rates. Conclusively, biochar application of 60 t/ha along with 270 kg N/ha is a promising option for improving soil quality and increasing photosynthesis, yield, and yield attributes of noodle rice.

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