Redox-driven colloidal mobility and its effects on carbon cycling in temperate paddy soils

<p>Rice paddy soils are known to represent a large proportion of global terrestrial carbon (C) stocks (ca.10 Pg), accumulating organic C in the topsoil due to cultivation under submerged conditions. Apart from the limited mineralization under anoxic soil conditions resulting from frequent field flooding, other mechanisms involving the dynamic interactions between organic C and redox-active minerals particularly Fe (oxy)hydroxides, together with the transport of organic C to deep mineral horizons, can lead to long-term C stabilization. Our previous studies have shown that up to 30-50 g m<sup>-2</sup> of dissolved organic C (DOC, defined as <450 nm) and 25-40 g m<sup>-2</sup> of Fe<sup>2+</sup> may be mobilized and translocated into the subsoil over a rice cropping season in temperate rice paddies, contributing to an increase in belowground C stocks. However, little is yet know on influence of frequent redox fluctuations on the contribution of colloidal organo-mineral associations to C mobilization and accrual in paddy subsoils.</p><p>We hypothesized that (i) redox fluctuations may lead to an overall increase in colloid dispersion (via reductive dissolution of Fe oxides, changes in soil pH, as well as neoformation of colloidal organo-mineral associations), and that (ii) colloidal mobility may represent an important C input to paddy subsoils. In order to evaluate the effects of redox fluctuations on colloid dynamics in situ, water-dispersible fine colloids (WDFC) were isolated from soils collected from different horizons along two profiles opened in adjacent plots under long-term paddy (P) and non-paddy (NP) management in NW Italy. Moreover, WDFC were also isolated from anaerobically-incubated topsoil samples to evaluate the changes in colloid dispersion under reducing conditions as a function of management. Colloidal size-fractionation and their elemental compositions were evaluated by asymmetric flow field-flow fractionation (AF4) coupled with OCD or ICP-MS. </p><p>Our results evidenced that redox cycling favours colloidal stability in the topsoils, with a preferential dispersion of the smallest-sized colloidal C (<30 nm and 30-240 nm fractions), even though larger-sized colloidal C (>240 nm) contributes predominantly to the WDFC. Consequently, under long-term paddy management colloidal dispersion and transport along the soil profile were probably responsible for the lower amounts of colloidal C (and Fe) observed in the Ap topsoil horizons of P with respect to NP, as well as for the significant accumulation of colloidal C in correspondence with the Brd subsoil horizons just beneath the plough pan. These illuvial horizons were also particularly rich in small-sized (30-240 nm) colloidal Fe, Al and Si possibly due to mineral phase changes induced by redox fluctuations.</p><p>Our findings therefore indicate that downward mobilization of colloidal C associated with Fe (hydr)oxides (e.g. coprecipitates) or small aluminosilicate minerals, rather than dissolved organic C, may represent an important process driving organic C accrual in paddy subsoils. However, further insights are still required to entangle the contribution of the different mechanisms involved.</p>

Response of Soil C and N, Dissolved Organic C and N, and Inorganic N to Short-Term Experimental Warming in an Alpine Meadow on the Tibetan Plateau

Although alpine meadows of Tibet are expected to be strongly affected by climatic warming, it remains unclear how soil organic C (SOC), total N (TN), ammonium N(NH4+-N), nitrate N(NO3+-N), and dissolved organic C (DOC) and N (DON) respond to warming. This study aims to investigate the responses of these C and N pools to short-term experimental warming in an alpine meadow of Tibet. A warming experiment using open top chambers was conducted in an alpine meadow at three elevations (i.e., a low (4313 m), mid-(4513 m), and high (4693 m) elevation) in May 2010. Topsoil (0–20 cm depth) samples were collected in July–September 2011. Experimental warming increased soil temperature by ~1–1.4°C but decreased soil moisture by ~0.04 m3m−3. Experimental warming had little effects on SOC, TN, DOC, and DON, which may be related to lower warming magnitude, the short period of warming treatment, and experimental warming-induced soil drying by decreasing soil microbial activity. Experimental warming decreased significantly inorganic N at the two lower elevations,but had negligible effect at the high elevation. Our findings suggested that the effects of short-term experimental warming on SOC, TN and dissolved organic matter were insignificant, only affecting inorganic forms.

Effects of amendment of different biochars on soil enzyme activities related to carbon mineralisation

This study aimed to investigate the effects of different biochars on soil enzyme activities associated with soil carbon (C) mineralisation. Biochars were produced from two types of feedstock (fresh dairy manure and pine tree woodchip) at temperatures of 300°C, 500°C, and 700°C. Each biochar was mixed at a ratio of 5% (w/w) with a forest loamy soil and the mixture was incubated at 25°C for 180 days. Soil mineralisation rates, soil dissolved organic C, soil microbial biomass C, and five soil enzyme activities were measured during different incubation periods. Results showed that biochar addition increased soil enzyme activities at the early stage (mainly within the first 80 days) because biochar brought available nutrients to the soil and increased soil dissolved organic C and microbial activity. Soil enzyme activities were enhanced more by the dairy manure biochars than by the woodchip biochars (P < 0.05). The enhancement effect on enzyme activities (except catalase activity) was greater in the treatments with biochars produced at lower pyrolysis temperature (300°C). Linear relationships between some soil enzymes and C-mineralisation rates might indicate that the increased enzyme activities stimulated soil C mineralisation at the early stage. However, the biochar additions could result in great C sequestration in the long term, especially for the woodchip biochars pyrolysed at higher temperatures.

Carbon leaching from undisturbed soil cores treated with dairy cow urine

Solubilisation of soil carbon (C) under cow urine patches may lead to losses of soil C by priming or leaching. We investigated the solubilisation and bioavailability of soil C in undisturbed pasture soil treated with urine. We also studied the contribution of acid-neutralising capacity (ANC) forcing and aggregate disruption as mechanisms of soil C solubilisation. Undisturbed soil cores (0–5 cm; Typic Udivitrand) were treated with water or δ13C-enriched urine and subsequently leached. Urine deposition increased total C and dissolved organic C leaching by 8 g C m–2 compared with water. Soil C contributed 28.1 ± 0.9% of the C in the leachate from urine-treated cores (ULeachate). ANC forcing of urine was 11.8 meq L–1 and may have contributed to soil C leaching, but aggregate disruption was unlikely to have contributed. The bioavailability of organic C in ULeachate was four times greater than in both cow urine and water leachate. It is possible that ULeachate may lead to priming of soil C decomposition lower in the profile. Further testing under field conditions would determine the long-term contribution of urine deposition to dissolved organic C leaching and the fate of solubilised C in pastoral soils.

Effects of manganese addition on carbon release from forest floor horizons

Lignin concentration in organic residues largely controls their decomposition. Mn2+ may well play a key role in ligninolysis because it is a cofactor of manganese peroxidase, an enzyme of the lignin-degrading system. This study aims to investigate the effects of Mn2+ addition on forest floor horizon decomposition during laboratory incubation. Therefore, we sampled two distinct forest floors from European beech ( Fagus sylvatica L.) stands: a mor and a moder. Lignin and Mn concentrations in forest floor upper layer were significantly larger in moder than in mor. Three horizons from each forest floor were separately incubated with or without Mn2+ addition (250 mg Mn·kg dry matter–1) and the release of both CO2 and dissolved organic C was measured. The dissolved organic C release was not impacted by the Mn2+ addition, while a clear increase in CO2 release from specific horizons was observed. Our data suggest that the impact of the Mn2+ addition depends on (i) the forest floor type and on (ii) the organic matter decomposition stage.