Modeling aerobic decomposition of rice straw during the off-rice season in an Andisol paddy soil in a cold temperate region of Japan: Effects of soil temperature and moisture

Abstract

Submerged rice paddies are a major source of methane (CH 4 ) which is the second most important greenhouse gas after carbon dioxide (CO 2 ). Accelerating rice straw decomposition during the off-rice season could help to reduce CH 4 emission from rice paddies during the single rice-growth season in cold temperate regions. For understanding how both temperature and moisture can affect the rate of rice straw decomposition during the off-rice season in the cold temperate region of Tohoku district, Japan, a modeling incubation experiment was carried out in the laboratory. Bulk soil and soil mixed with 2% of δ 13 C-labeled rice straw with a full factorial combination of four temperature levels (−5 to 5, 5, 15, 25°C) and two moisture levels (60% and 100% WFPS) were incubated for 24 weeks. The daily change from −5 to 5°C was used to model the freezing-thawing cycles occurring during the winter season. The rates of rice straw decomposition were calculated by (i) CO 2 production; (ii) change in the soil organic carbon (SOC) content; and (iii) change in the δ 13 C value of SOC. The results indicated that both temperature and moisture affected the rate of rice straw decomposition during the 24-week aerobic incubation period. Rates of rice straw decomposition increased not only with high temperature, but also with high moisture conditions. The rates of rice straw decomposition were more accurately calculated by CO 2 production compared to those calculated by the change in the SOC content, or in its δ 13 C value. Under high moisture at 100% WFPS condition, the rates of rice straw decomposition were 14.0, 22.2, 33.5 and 46.2% at −5 to 5, 5, 15 and 25°C temperature treatments, respectively. While under low moisture at 60% WFPS condition, these rates were 12.7, 18.3, 31.2 and 38.4%, respectively. The Q 10 of rice straw decomposition was higher between −5 to 5 and 5°C than that between 5 and 15°C and that between 15 and 25°C. Daily freezing-thawing cycles (from −5 to 5°C) did not stimulate rice straw decomposition compared with low temperature at 5°C. This study implies that to reduce CH 4 emission from rice paddies during the single rice-growth season in the cold temperate regions, enhancing rice straw decomposition during the high temperature period is very important. ARTICLE HISTORY

Comparison of fallow season CO<SUB>2</SUB> efflux from paddy soil estimated using laboratory incubation with eddy covariance-based flux

“…The soil CO 2 emission rates in most previous incubation studies of soils in upland agricultural fields determined the maximum values at soil water contents between 60 and 90 of the field capacity, and they decreased at higher soil water contents. By contrast, our results, as well as those of Nakajima et al 2015 , obtained maximum values at 100 of the field capacity. These results may show general characteristics of paddy soils during FS.…”

“…The temperature sensitivity of the soil CO 2 emission rates obtained in our study as Q 10 was higher than 1.81 at a lower moisture level and 1.56 at a higher moisture level, as shown in Table 3 from Nakajima et al 2015 . This may be attributed to the shorter incubation period in our study 38 days compared with that used by Nakajima et al 2015 , which was equivalent to the whole FS. The Q 10 values obtained were within the ranges for various soil types e.g., Davidson and Janssens, 2006;Moriyama et al, 2013 .…”

“…However, a substantial number of their flux data were discarded through quality tests and then filled with estimations, which could result in a large source of uncertainty. Nakajima et al 2015 investigated the carbon balance in paddy soils under aerobic conditions using soil samples obtained after the harvest by three methods: CO 2 evolution, amount of soil organic matter, and the addition of a high ratio of 13 C organic carbon to incubating soil samples. They obtained a higher CO 2 evolution rate at the field capacity than that at 60 of the field capacity.…”

Q10 values vary with different kinetic properties of C mineralization

“…However, the Q 10 values derived from various experimental methods can be confounded by the different qualities and retention times of the co-existing labile and recalcitrant components (Conant et al, 2008;Hamdi et al, 2013;Nakajima et al, 2016;Wang et al, 2013). For instance, the "equal-time" approach always leads to great discrepancies in the degradation of fast-vs. slow-turnover C pools among samples along with increasing incubation temperature and time, while the "equal-C" approach can underestimate the contribution of more recalcitrant substrates (Hamdi et al, 2013).…”

“…Thus, the kinetic parameters of various soil C pools should be fitted to the observational data to determine the temperature sensitivity of the C mineralization, including the rate constants and the relative sizes of the C pools of varying decomposability (Davidson and Janssens, 2006). Furthermore, the temperature sensitivity of C mineralization not only varies in those kinetic properties, which are partially determined by soil properties , but also changes with environmental conditions, such as constant vs. varying temperature regimes (Conant et al, 2008;Fang et al, 2005;Nakajima et al, 2016;Xia et al, 2009;Zhu and Cheng, 2011), the aeration statuses (Blagodatskaya et al, 2014;Devêvre and Horwáth, 2000;Diakova et al, 2016), the forest stands (Guo et al, 2016;Gutiérrez-Girón et al, 2015;Rey and Jarvis, 2006), and the soil horizons (Laganière et al, 2015;Xu et al, 2014). These constraints, especially the temperature and the aeration status, are fixed as constant in most studies, and a continuously changing vs. constant environmental comparison has rarely been scrutinized (Conant et al, 2008;Hartley et al, 2008;Pettersson and Bååth, 2003;Ranneklev and Bååth, 2001).…”