Effects of cropland conversion on OC pool in bulk soil
Cropland restoration identified as an efficient ecological project to promote soil C sequestration in karst erosion areas28,30. The conversion from MS to FG resulted in the total soil OC content and stock across 0–30 cm layers increasing by 46.12% and 43.73% respectively. The result was highly coincident with previous studies observed at 0–10 cm layer, which reported that FG cultivation replaced from MS cultivation could remarkably increase soil OC pool in karst region, Southwest China28. In our study, the lower OC content and stock in MS may be partially attributed to the non-returned crop residues and increased exposure of deep soil OM to oxygen under tillage disturbance, resulting in decreased soil OC accumulation through reducing the input of OM and accelerating OM decomposition28,30,37,38. Nevertheless, the conversion from MS to FG can increase the soil OC pool by increasing inputs from crops. For detail, laregly aboverground crops are harvested and removed from the fields each every year for economic production, there is thus a lack of aboverground OC input. Therefore, the root biomass became the main source of OM inputs, and even slight changes in biomass can substantially alter soil C level39. In the present study, the root biomass in FG field was approximately 6 times that in MS field (110.06 ± 17.24 kg hm−2 averagely) (Table S2). Consequently, the higher root biomass in FG are responsible for the corresponding higher C storage of fine root in FG, which is supported by the fact that higher amount of C were stored in the fine roots of FG field compared with that of MS field (Table S2). In fact, several studies have demonstrated that cultivation of perennial grasses is efficient in stimulating soil OC accumulation owing to its great amount of fine roots and underground biomass33,40. Soil disturbance (such as tillage) is one of the main causes of soil C depletion in agricultural systems, and increased tillage practice can result in greater soil C loss41,42,43. Therefore, the frequent tillage conducted in MS field resulted in lower levels of OC than that in FG field under minimal tillage disturbance.
Impacts of cropland conversion on soil aggregates structure and stability
Soil structure plays an important role in soil environment and quality, which is strongly characterized by soil aggregates and their stability43,44. In our study, soil macro-aggregates dominated the largest portion of total soil while meso-aggregates and micro-aggregates were only accounted for a small portion, indicating that cropland conversion could facilitated the formation of macro-aggregates (Table 2). These findings are in line with other studies, wherein that macro-aggregates occupied the major portion of total soil following farmland or vegetation restoration19,30. Tillage disturbance often disrupts aggregates by bringing subsurface soil to the surface, which can readily promote soil C turnover and hinder macro-aggregate formation45. Conversely, minimal tillage experienced and greater accumulation of root residues resulted in higher C accumulation in the FG field. Furthermore, fine roots improved the soil aggregate stability via the interaction with mycorrhizal fungi, which produced exudates and binding agents and promoted the formation of soil aggregates46,47. Therefore, higher inputs of root residue in the soil could enhance the capacity of aggregate re-formation. In fact, these can be supported by the higher value of root biomass and its C stock in the FG field. In addition, forage grass cultivation can enhance the formation of large and stable soil aggregates by fine roots and fungal hyphae through the production of exudates and binding agents, such as humic compounds, polymers and roots48,49. Thus, few tillage disturbance and higher inputs of root biomass in FG field resulted in soil aggregation enhanced, especially macro-aggregates.
Soil aggregate stability can also be characterized by the values of MWD and GMD. Higher MWD or GMD values indicate greater aggregate stability due to more agglomerate ability. The value of MWD in the current study varied from 1.36 to 1.96, which was classified as “stable” by LeBissonnais’ categorization of aggregate stability50.
Regardless of soil depth, the FG field had the greatest MWD and GMD values, indicating that its soil aggregates were more stable than those of the other three cropland use types. We may thus draw the conclusion that FG cropland conversion can improve the stability of aggregates based on MWD and GMD.
Changes in OC stocks associated –aggregates following cropland conversion
Cropland use change generally affects soil C sequestration through changing OM inputs and decomposition19. Our study revealed that aggregate-associated OC was significantly higher in FG field than in MS field. These increases were mainly attributed to the new C derived from root residues inputs and decreased losses of OC associated-aggregate by C mineralization in FG soil49. Generally, tillage can breakdown large aggregates into small aggregates, and thus decrease the formation of soil macro-aggregates41,42. Thus, the lower OC content and stock associated-aggregate in MS field can be attributed to the OC loss resulting from soil erosion, and OM input reduction with tillage disturbance8,30,45.
In this study, the effects of cropland conversion on OC content associated-aggregate fractions occurred in the top 20 cm soil layers. In the karst region, approximate 57–89% of crop roots are concentrated in the surface soil layer, which directly affects OM inputs from underground root residues51,52. Meanwhile, tillage practices also happened on top 20 cm soil layer6,28,29. As a result, in soils below 20 cm, little or no tillage disturbance and limited OM inputs resulted in fewer or no distinctly changing levels of OC content associated with aggregate following cropland use change.
Cropland use change not only affected the OC stocks in bulk soil, but also affected the OC stocks associated-aggregates (Table 1). The difference of sensitivity of OC associated-aggregate to cropland use change may affect its contribution to bulk soil OC accumulation30,38. In our study, the macro-aggregate fraction was the most important contributor to total OC stock increase, followed by meso-aggregate and micro-aggregate (Fig. 4). This is primarily due to the higher amount and OC content of macro-aggregates. Overall all cropland use types, the OC stock associated with macro-aggregate in FG field was higher than that in other three cropland types regardless of soil depth (Fig. 4). For instance, OC stocks within macro-aggregate accounted for about 85.40%, 77.72% and 97.55% of total soil OC stock at 0–10 cm, 10–20 cm and 20–30 cm, respectively, under the conversion from MS to FG. Thus, the accumulation pattern of bulk soil OC stocks could closely related with changes of OC stocks associated with macro-aggregate under cropland use change.
The physical protection of OC in aggregates is regarded as one of the main mechanisms for soil OC accumulation through diminishing soil OC degradation and preventing its interaction with mineral particles53,54. In the present study, OC stock in bulk soil correlated substantially with the OC content-associated aggregate following cropland conversion (Fig. 5). Further analysised revealed that OC stocks in bulk soil was significantly correlated to OC stock associated with macro-aggregate (R2 = 0.83, p < 0.01), confirming that macro-aggregates are the major contributor to bulk soil OC accumulation where most OC stocks contained (Fig. 5). These may be caused by the binding agents of macro-aggregate, such as fungal hyphae, mycorrhizal hyphae, bacterial cells, and algae, which are highly dependent on soil OC and develop simultaneously with crop growth and build up a visible organic skeleton to enmesh the mineral particles by adsorption to form young macro-aggregates15,55,56. These patterns can proven by the shift in the increment of soil OC stocks following cropland conversion. Our calculation showed that the incrementary ratio of OC stock associated with macro-aggregate was more than 58% when MS replaced by FG (Table S3). However, the increment of OC stock associated with macro-aggregate in SG and MB field was relatively smaller and even shows a declining trend (Table S3). Based on the theory of hierarchical aggregation, OC stock increases with increasing aggregate size because of larger aggregates are composed of small particles plus organic binding agents15,29. Therefore, OC stocks in macro-aggregate were higher than those in other size fractions (Fig. 4). Overall, our findings demonstrate that OC accumulation was mainly due to the contribution of macro-aggregates. In the title of Table S3, the unit “Mg/hm2” changed “Mg/hm2“.
Changes of OC stocks within aggregates are mainly attributed to two factors: (1) changes in the mass of special aggregate fractions and (2) changes in the OC content associated-aggregate fractions19,20. Thus, a better understanding of the dynamic of soil OC pool response to cropland use change is needed to character the changes of mass and OC content within special soil aggregates. Based on our calculation, the contribution pattern of aggregate mass and its OC content to the net accumulation of OC stock within aggregates differed substantially among four cropland use types. Specifically, the increase in OC stock within macro-aggregate was primarily attributed to increases in the OC contents of macro-aggregate and decreases in the mass of macro-aggregate fraction when MS converted to FG (Table 3). Prior work in karst area have reported that a relative higher recovery of soil aggregate structure and larger increase in macro-aggregate amount following vegetation restoration, indicating that OC accumulation relies on a well-developed soil structure30. These changes were also supported by the fact that the value of F1was greater than F2 in macro-aggregate when MS converted to FG (Table 3). Nevertheless, slight decreases in both mass and its OC content associated-aggregate fractions result in few or no changes in OC stock of macro-aggregate when MS converted to SG or MB (Table 3). Therefore, we can conclude that MS converted to FG can effectively improve the bulk soil OC stock in the karst region of southwest China. The OC stocks associated with macro-aggregate contribute to the most to increase bulk soil OC accumulation, which depends on the OC content within macro-aggregate following the conversion from MS to FG.
Source: Ecology - nature.com
