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Purpose: The applications of biochar (BC) and polyacrylamide (PAM) may have interactive effects on carbon (C) dynamics and sequestration for improving the soil quality and achieving sustainable agriculture. Relative to BC and PAM, rhizodeposits act as C and energy source for microorganisms and may change the mineralization dynamics of soil organic matter (SOM). No attempt has been made to assess the effects of BC, anionic PAM, or their combination on the decomposition of different aged C-labeled rhizodeposits. The objective of this study was to investigate the effects of the treatments mentioned above on the decomposition of different aged C-labeled maize rhizodeposits. Materials and methods: biochar (BC) at 10 Mg ha or anionic PAM at 80 kg ha or their combination (BC + PAM) was applied to soils with/without 2-, 4-, 8-, and 16-day-aged C-labeled maize rhizodeposits. After that, the soil was incubated at 22 °C for 46 days. Results and discussion: After 2 days of incubation, the total CO efflux rates from the soil with rhizodeposits were 1.4-1.8 times higher than those from the soil without rhizodeposits. The cumulative CO efflux (32 % of the C input) was maximal for the soil containing 2-day-aged C-labeled rhizodeposits. Consequently, 2-day-aged rhizodeposits were more easily and rapidly decomposed than the older rhizodeposits. However, no differences in the total respired CO from rhizodeposits were observed at the end of the incubation. Incorporation of C into microbial biomass and 66-85 % of the C input remained in the soil after 46 days indicated that neither the age of C-labeled rhizodeposits nor BC, PAM, or BC + PAM changed microbial utilization of rhizodeposits. Conclusions: Applying BC or BC + PAM to soil exerted only minor effects on the decomposition of rhizodeposits. The contribution of rhizodeposits to CO efflux from soil and MBC depends on their age as young rhizodeposits contain more labile C, which is easily available for microbial uptake and utilization. [ABSTRACT FROM AUTHOR]

• High microbial activity in topsoil causes apparent PE shortly after substrate addition. • The input of C from fertilizers induces apparent PE at the 10–30 cm depth. • The release and decomposition of mineral-bound organic C shortly after substrate addition contributes to positive PE. • Negative PE in topsoil during later incubation stages is linked to reduced microbial biomass and substrate depletion. • In subsoil, positive PE and net C loss was caused by microbial N mining. Fertilization is crucial for increasing crop productivity and it alters soil microbial biomass and activities. These alterations exert implications for soil carbon (C) stocks, primarily through the priming effect (PE). Here, we investigated the mechanisms underlying PE and their impact on soil C stocks in paddy soils subjected to long-term (31 years) fertilization. Soils from four depths within the upper 40 cm layer, subjected to four fertilization types (no fertilizer, mineral fertilizer, mineral + straw, mineral + chicken manure), were incubated for 60 days with or without 13C-labeled glucose. Following substrate addition, an increase in isotopically unlabeled CO 2 efflux may stem from accelerated microbial turnover (apparent PE) or modified soil organic matter (SOM) mineralization (real PE). By analyzing microbial biomass (MBC), CO 2 and dissolved organic C (DOC) dynamics, we explored the partitioning of C from glucose into three pools. Following glucose addition (day 2), the SOM-derived MBC decreased by 4.7 % at 0–10 cm in unfertilized and by 6.3 % in mineral-fertilized soils. SOM-derived MBC also decreased in soils fertilized with straw- and chicken manure at 0–20 and 0–30 cm. Hence, the apparent PE closely correlated with fertilization types. At day 2, the SOM-derived DOC increased with the increase in extractable Fe2+ content, indicating high contribution of the released mineral-bound organic C in the DOC pool. Microbial decomposition of SOM-derived DOC and loss of unlabeled C through apparent PE increased the release of unlabeled CO 2 , resulting in a positive but low PE. After 60 days of incubation, the addition of glucose to the subsoils (30–40 cm) caused a net loss of C (positive PE) of 6.6–19 mg kg−1. In contrast, negative PEs were observed in the topsoil, indicating a decrease in microbial activity and C turnover. These findings suggested that fertilization regimes, whether involving long-term extra C input or not, can alter microbial biomass and turnover rates. Moreover, the C turnover mechanisms in paddy soils are complex, including apparent PE, release of mineral-bound organic C, and negative PE. Collectively, these processes can consequently change soil C stocks. [ABSTRACT FROM AUTHOR]

Meadows and forests are the main vegetation types in temperate terrestrial ecosystems, and largely contribute to soil carbon (C) stock. Bioavailable C inputs can accelerate microbial decomposition of soil organic matter (SOM), which is known as "priming effect". However, it is still unclear how priming effect, as an important mechanism influencing soil C sequestration, is influenced by spatial transition of vegetation from meadow to forest. To investigate the mechanism of priming effect along a spatial transition gradient of vegetation, a soil incubation experiment with 14C labeled glucose was combined with microbial rDNA sequencing and gene composition prediction. The results showed that with the vegetation transition from meadow to forest soil available phosphorus (P) significantly increased, in contrast to dissolved nitrogen (N) and C which remained unaffected. Moreover, the soil microbial community composition shifted towards a higher relative abundance of K-strategists (Acidobacteria) and a lower abundance of r-strategists (Actinobacteriota) along the vegetation transition from meadow to forest. In the meadow, the microbial community consumed more of the added glucose and increased priming effect. This was accompanied by lower available P but higher soil bacterial gene function encoding P cycling. In contrast, increased soil P availability in forest soils caused a decelerated microbial metabolism of phosphorylated organic compounds within microbial biomass due to decreased microbial demand for P acquisition from SOM, and thus resulted in suppression of the priming effect. Our study showed that P availability and microbial community shifts in spatial transition zones between meadows and forests are important drivers for the priming effect on SOM decomposition. • 14C radioactive isotope labeling was applied in combination with microbial rDNA sequencing. • Soil microbial community in meadow consumed more of the added glucose than that in forest. • Vegetation transition from meadow to forest suppressed the priming effect. • P availability and microbial community shifts are important drivers for the priming effect. [ABSTRACT FROM AUTHOR]

• The effect of long-term fertilization practices on soil carbon (C) was assessed. • Fertilizations increased soil C despite the induced positive priming effects (PEs). • Manure application greatly increased soil C when combined with mineral fertilizer. • The magnitudes of PEs under the treatments were explained by microbial nitrogen mining. Organic and synthetic fertilizers not only increase soil fertility and crop productivity but also enhance soil organic carbon (SOC). However, the priming effect (PE) leads to increased soil carbon (C) loss through native SOC mineralization. To date, the mechanisms by which long-term (>66 years) synthetic and/or organic fertilization alters net SOC sequestration remain a matter of debate. This study aimed to assess the effects of different fertilization practices on SOC decomposition and PE in agricultural systems subjected to long-term annual synthetic and/or organic fertilizer application. This aim was achieved by collecting topsoil samples (0–20 cm) from four long-term fertilization practices, i.e., unfertilized, synthetic supplemental (+s), cattle farmyard manure (+m, similar nutrient amount to +s), and synthetic fertilizer with farmyard manure (+s +m, the highest nutrient amount). The soil samples were incubated for 33 days with and without 13C-glucose addition, and a CO 2 isotope analyzer combined with a modeling approach was used to establish a real-time method to monitor CO 2 and 13CO 2 production rates during the incubation period. Overall, +m increased the cumulative SOC-derived CO 2 (SOC-CO 2) by 107, 74, and 24 % compared to the unfertilized, +s and +s +m, respectively. The higher SOC-CO 2 in +m treatment was associated with the greatest priming effect (PE, 390 ± 21 mg C kg soil−1), which corresponded to a 30 % increase compared to the average of the treatments that involved synthetic fertilizer (+s and +s +m) and a 137 % increase compared to the unfertilized control. The results were explained by the lower dissolved nitrogen (N), a proxy of available mineral N, in +m compared to +s +m, thus enhancing microbial mining for additional N via increasing SOC mineralization. However, the combined application of synthetic fertilizer and manure in the +s +m treatment provided enough easily accessible nutrients for microbial growth and activities from the applied synthetic fertilizer, leading to lower SOC mineralization than manure (+m) alone. Nevertheless, the treatments with manure application (i.e., +m and +s +m) significantly increased net SOC compared to the synthetically fertilized treatment and unfertilized control, suggesting greater C inputs than outputs and leading to high SOC accumulation over time. These results indicated that organic manure has a great potential to mitigate climate change by increasing SOC over time, which can be fostered by the addition of synthetic fertilizer; however, caution still needs to be taken regarding the quality and quantity of the added fertilizer. [ABSTRACT FROM AUTHOR]