Correction to: Continuous maize cropping accelerates loss of soil organic matter in northern Thailand as revealed by natural 13C abundance


 AimsThe loss of soil organic matter (SOM) has widely been reported in the tropics after changing land use from shifting cultivation to continuous cropping. We tested whether continuous maize cultivation accelerates SOM loss compared to upland rice and forest fallow. Methods: Because litter sources include C4 plants (maize in maize fields and Imperata grass in upland rice fields) in Thailand, C3-derived and C4-derived SOM can be traced using the differences in natural 13C abundance (δ13C) between C3 and C4 plants. We analyzed the effects of land use history (cultivation or forest fallow period) on C stocks in the surface soil. Soil C stocks decreased with the cultivation period in both upland rice and maize fields. ResultsThe rate of soil organic carbon loss was higher in maize fields than in upland rice fields. The decomposition rate constant (first order kinetics) of C3-plant-derived SOM was higher in the maize fields than in the upland rice fields and the C4-plant-derived SOM in the forest fallow. Soil surface exposure and low input of root-derived C in the maize fields are considered to accelerate SOM loss. Soil C stocks increased with the forest fallow period, consistent with the slow decomposition of C4-plant-derived SOM in the forest fallows. ConclusionsContinuous maize cultivation accelerates SOM loss, while forest fallow and upland rice cultivation could mitigate the SOM loss caused by continuous maize cultivation.


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In Southeast Asia, traditional shifting cultivation has been replaced by continuous cropping 40 systems (Kyuma and Pairintra, 1983). In northern Thailand, upland rice is cultivated between 41 forest fallow periods for subsistence, but continuous maize cultivation has increased along 42 paved roads with infrastructure improvement (Bruun et al., 2017). Intensive agriculture without 43 organic amendment leads to a loss of soil organic matter (SOM), which is essential for  Because SOM gain or loss is dependent on the balance between C inputs (mainly litter 57 inputs) and outputs (heterotrophic respiration, leaching, and erosion), the effects of land use on 58 SOM stocks can be quantified using annual soil C budgets (Fujii et al., 2009) or long -term 59 monitoring of soil organic carbon (SOC) stocks (Fujii et al., 2019(Fujii et al., , 2020. Alternatively, SOC 60 stocks can be compared between sites that share soil attributes with different land-use histories 61 (cultivation or fallow periods). Although our study site is in a remote part of northern Thailand, 62 its land-use history has been monitored continuously (Sakai, 2005; Fig. 1 (Sakai, 2005). After the war, northern Thai (Khon Muang) and Hmong migrated in 84 1982 and 1987, respectively, and started cultivating upland rice and maize in the upstream forest 85 (Fig. 1a). After the village protected the upstream forest for water security and a paved highway 86 was constructed in 1995, intensive maize cultivation expanded to the fields close to the highway 87 ( Fig. 1a,b). The population increase and influx of refugees increased maize production. The 88 land use history has been recorded in the village (Fig. 1a,b). The cultivation and fallow periods 89 of the sampling locations were determined by interviews with farmers and a field survey, cross-  The SOM loss data were fitted to a single exponential decay function: where Rr and Ri are the initial and remaining C stock (Mg C ha −1 ) respectively, k is the 128 decomposition rate constant (yr −1 ), and t is the cultivation or fallow period (yr).  (Table 1; Fig. 2a). By contrast, the soil C stocks were negatively correlated with the 144 cultivation period in both the upland rice and maize fields (Fig. 2b,c). Comparison of the slopes 9 of the linear regressions indicated that the SOC loss rate was significantly ( P < 0.05) higher in 146 the maize field than in the upland rice field (Fig. 2bc).

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The δ 13 C values of Imperata grass and maize litter (C4 plants) differed significantly 148 from those of forest litter (C3 plants) (Table 1). This supports the precondition of our study that 149 C3 plant-derived and C4 plant-derived SOM can be traced in soils. The C/N ratios and C 150 concentrations were significantly (P < 0.05) lower in the heavy fractions than in the light 151 fractions ( Table 2). The δ 13 C values in the heavy fraction were significantly (P < 0.05) higher 152 than in the light fraction in the forest soils, but no significant differences in the δ 13 C values 153 were found between the heavy and light fractions in the upland rice and maize fields (Table 2). proportion of C4-plant-derived C in the light fractions increased from 10% to 38% in the maize 162 fields and from 8% to 24% in the upland rice field (Table 2). Similarly, the proportion of C4 -plant-derived C in the heavy fractions increased from 3% to 33% in the maize fields and from 164 11% to 32% in the upland rice field ( Table 2). The C3-plant-derived C stock decreased with 165 cultivation period in the maize (Fig. 4a) and upland rice (Fig. 5a) fields. However, the 166 proportion of C4-plant-derived C in the heavy fraction decreased from 30% to 6% in the forest 167 fallow ( Table 2). The total C stock in the soil increased with the fallow period in the forest 168 fallows, but the C4-plant-derived C stocks decreased (Fig. 5b).

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Fitting the SOM loss data to an exponential decay gives the decomposition rate 170 constants or k values, except for the light fraction of the forest sites (Table 3; Figs. 4a, 5a,b).

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The k value of the heavy fraction was significantly (P < 0.05) lower than that of the bulk soil 172 in the forest fallow sites (Table 3). By comparison, the k value of the heavy fraction was 173 significantly (P < 0.05) higher than that of light fraction in both the upland rice and maize fields 174 (   (Table 3). The SOM in the heavy fraction has higher δ 13 C values and lower C/N ratios 183 due to selective respiration of 12 C relative to 13 C (

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In the maize field, C3-plant-derived C in the heavy fraction decomposes faster than in 186 the light fraction (Table 3)

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The slower decomposition of C3-plant-derived SOM in the upland rice fields compared 208 to the maize fields (Table 3) is consistent with the lower SOM loss rates in upland rice fields 209 (Fig. 2b). As in the maize field, C3-plant-derived SOM is not equal to forest-derived SOM in 210 the upland rice fields, as upland rice-derived SOM could also be provided. Based on the lower 211 shoot/root ratios of upland rice, root-derived C inputs would be greater in the upland rice fields 212 than in the maize fields (Kondo et al., 2000). This could mitigate the loss of C3 -plant-derived 213 SOM in the upland rice fields (Fig. 5a). The limited erosion due to rice straw covering the so il 214 surface could also mitigate SOM loss.

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The main source of C4-plant-derived SOM in the forest fallow is Imperata grass, as 216 upland rice was traditionally cultivated before forest fallow started (Sakai, 2005

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However, the decomposition of C4-plant-derived SOM in the forest fallow is much slower than 219 the decomposition of C3-plant-derived SOM in the croplands in our study (Table 3) (Table 1; 240 Figs. 4c, Fig. S2c) compared to 16.2% of the annual litterfall C input in the forest fallow soils 241 (Table 1; Fig. 2a). Judging from the finding that soil C accumulation rates in the forest fallow 242 exceed SOM loss rates in the cropland soils (Fig. 2), forest fallow has high potential to mitigate             Changes in stocks (0-5 cm) of (a) C4-plant-derived C (bulk soil) in the forest fallows and (b) C3-plantderived C (bulk soil) in the upland rice elds. The curves represent tting with single exponential decay function.

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