Cropland restoration has been reported as one of the most effective measures for soil total nitrogen (N) accumulation in karst degraded regions. However, the patterns of N associated-aggregate and its contribution to net soil N accumulation following cropland conversion remain poorly understood. The experiment included four treatments with one control and three restoration strategies, that is: maize-soybean rotation cultivation (control), sugarcane, mulberry and forage grass cultivation. Soil samples were selected to determine the soil aggregate amount and its associated N content and stock across 0–30 cm soil layer. Macro-aggregate (>2 mm) was the predominant aggregate fraction in all cropland use types and had the highest N stock. Forage grass cultivation substantially enhanced total N stock in bulk soil and aggregate fractions. Bulk soil N stocks were positively correlated with N content and stock aggregate-associated. The increase in N stock in forage grass soil was primarily attributed to the increased N stock within macro-aggregates, which is further attributed to the increased N content within macro-aggregates. Soil C:N ratio in macro-aggregate, 2–1 mm and micro-aggregate (<0.25 mm) but not in bulk soil was significantly changed following cropland conversion. Overall, forage grass cultivation replaced maize-soybean cultivation was suggested as an ecological restoration model to enhance soil N sequestration potential, owing to its role in increasing N stock of aggregate in the karst degraded region of Southwest China.
Research Article
Difference in total N and its associated-aggregate N following cropland restoration in a karst region, Southwest China
https://doi.org/10.21203/rs.3.rs-1966596/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 17 Feb, 2023
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N accumulation
aggregate
cropland restoration
N stock
macro-aggregate
Nitrogen (N) is an essential element for plant growth and development, and is often the primary limiting resource for productivity of many terrestrial ecosystems on a global scale (LeBauer &Treseder 2008). Soil N plays a pivotal role in improving soil fertility and providing plant nutrients by directly or indirectly altering microbial activities and nutrient availability. Furthermore, the accumulation of soil carbon (C) is strongly dependent on the input of added N, and hence soil N availability is often regarded as a major limiting factor affecting long-term soil C sequestration after ecological restoration (Wen et al. 2016，Liu et al. 2019). As a result, it is undoubtedly imperative to explore N dynamics following cropland restoration in order to develop new effective restoration model to mitigate global warming and improve soil quality.
Soil N pool can either increased, decreased or negligible changes following cropland use change (Gelaw et al. 2015，Deng et al. 2016, Zhang et al. 2019, Lan et al. 2020). The inconsistency of these results indicates that soil N dynamics are complex, which is affected by climate, restoration stages, land management, fertilizer, tillage and soil properties ( Kasper et al. 2009, Johnson et al. 2014, Gelaw et al. 2015, Xiao et al. 2017, Zhu et al. 2018, Sokolowski et al. 2020, Weidhuner et al. 2021, Zhong et al. 2021). Therefore, there is considerable unclear information on soil N accumulation, and the mechanisms that explain the soil N dynamics after vegetation restoration are insufficiently understood.
Soil aggregates are the fundamental units of soil structure, and considered to be an important carrier in stabilizing and sequestrating soil C and N (Gupta Choudhury et al. 2014). Soil aggregates protect the soil organic matter (SOM) against decomposition by trapping and minimizing the direct contact of SOM with the soil microorganisms (Tisdall and Oades 1982). Thus, soil aggregate is a vital index for soil C turnover and for the cycling of nutrients, particularly N (Jiang et al. 2011, Zhu et al. 2017, Zhang et al. 2019, Schlüter et al. 2020). Different aggregate sizes have different abilities to maintain and supply soil N, and then resulted in the difference distribution of N associated with various aggregate fractions (Xiao et al. 2017, Weidhuner et al. 2021, Zhong et al. 2021). Generally, macro-aggregate created from micro-aggregate through the process with transient and temporary organic binders. These binders are mainly distributed in plant roots, fungal hyphae and polysaccharides of microbial and plant origin (Li et al. 2019). A decrease in macro-aggregates stability can be caused by the physical disturbance of soil (Xiao et al. 2021). Hence, it is imperative to understand the effects of soil aggregates on soil N accumulation following cropland restoration
The karst system in Southwest China, is one of the most ecologically vulnerable regions due to extensive human activities on extremely fragile geological background (Jiang et al. 2014). The soil in this region have been degraded and eroded, following the conversion of natural vegetation into cropland (Li et al. 2017). Maize-soybean rotation (MS) cultivation was largely implemented in the karst region, which caused severe SOM loss, including N loss (Wen et al. 2017, Li et al. 2018a). Therefore, several emerging ecological restoration cropland use types, such as sugarcane (SG), mulberry (MB) and forage grass (FG) cultivations, have been developed to replace maize-soybean rotation in recent years. Numerous studies have largely concentrated on the on soil physical properties (Yao et al. 2009), soil quality (Zhang et al. 2019b), soil microbial community (Liang et al. 2016, Xiao et al. 2020) and soil C and N content (Xiao et al. 2017a, Hu et al. 2018, Liu et al. 2019b) following cropland restoration in this region. Nevertheless, little is known about soil aggregate fractions and their contribution to bulk soil N accumulation following cropland restoration. Previous studies also have disclosed that FG cultivation replaced from MS distinctly enhanced soil organic carbon (OC) and N pool in karst region (Li et al. 2018a) and soil OC accumulation was largely attributed to the increase of OC stock associated with macro-aggregate (Xiao et al. 2021). Further research indicated that macro-aggregates generally prefer higher soil C content than micro-aggregates following vegetation or cropland restoration (Hu and Lan 2020, Xiao et al. 2021). The soil OC stocks within aggregates were predominantly determined by the amount of aggregates and its associated OC content (Wei et al. 2013, Zhong et al. 2019). Soil C and N was coupled change in soil ecosystem (Zhao et al. 2021). However, the karst region is characterized by strong heterogeneity and fragmentation (Wang et al. 2019). As a result, we cannot assure that the dynamic of soil N associated with aggregate may be similar to soil OC change in the karst region following cropland use change. Thus, the objective of this study was (1) to determine the distribution of N associated with aggregates following cropland conversion, (2) to determine whether the changes in N stocks aggregate-associated were primarily due to changes in the mass of aggregate fraction or to the changes in the N content associated-aggregate.
Site description
This study was conducted in Huanjiang County (24°44′–25°33′N, 107°51′–108°43′E) of Guangxi Zhuang Autonomous Region, southwest China. The detailed climate information was presented elsewhere (Li et al. 2018a). The karst areas are characterized by gentle valleys embraced by hills and the soil is calcareous leptosols (limestone soil) according to the FAO/UNESCO classification system (Anon, 1974).
The experiment adopted a randomized complete block design with four blocks (about 9 km2). With each block, one plot (greater than 400 m2 each) was selected for the fields of MS, mulberry, MB, SG and FG cultivations. The latter three fields were converted from MS cultivation and had been under the same crop management for about 15–20 years. The cropland use types were managed with typical practices of the region. All of the information, i.e., crop species, tillage frequency, harvest frequency, fertilization rate and replanting frequency, have been described as by Li et al. (2018a). In addition, the planted species in the forage grass field was hybrid Napier grass (Pennisetum purpureum Schumach × P. amreicanum (L.) Leeke). This grass is commonly considered as a perennial fodder crop mainly due to its characters, i.e., fast growth rate, drought tolerance, high productivity and protein content. All plots were distributed over the valley or bottom slopes. In total, 16 plots were included with four plots for each of the four cropland use types.
Soil sampling and analysis
Soil samplings were conducted in October 2019. In each plot, after possible litters were removed, we collected four soil profiles (0–30 cm) randomly. Each profile was divided into three horizons, that is, 0–10 cm, 10–20 cm and 20–30 cm. In addition, two pits of 60×60×30 cm were dug to collect samples using metal rings for the measurement of bulk density at three soil layers. The undisturbed soil in the boxes was brought back to the lab and gently divided the soil into two portions for analysis, including (1) one was passed through 2 mm sieve for determining N content of bulk soil and (2) aggregation fractions and their N contents.
Aggregate fractionation
The soil aggregate fractions were determined by dry sieving methods, as described by (Gartzia-Bengoetxea et al. 2009). Briefly, 100 g of air-dried soil samples were taken on a nest of sieves (2 mm, 1 mm, 0.5 mm and 0.25 mm), then agitated for 90 s with a sieve shaker at 600 oscillation min-1. The soil retained on each sieve and in the bottom container was collected under 0.25 mm sieve. The soil aggregates were separated as macro-aggregates (>2mm), meso-aggregates (2–0.25 mm), and micro-aggregates (<0.25 mm). To determine the mass distribution of soil fractions, the sub-sample of soil fractions were oven dried at 105 ℃ and weighted at room temperature. A special aggregate sample and bulk soil was ground to pass through a 100 mesh sieve for the measurement of OC and N contents as described previously (Wen et al. 2016, 2017).
The N stock (Mg hm-2) in bulk soil was calculated using the following equation:
Where D is the thickness (cm) of the soil layer, BD is the bulk density (g cm-3) and N is the content (g kg-1).
The N stock (Mg hm-2) in aggregates fractions was calculated using the following equation:
Where Mi is the amount of soil in the ith size fraction (kg m-2) and Ni is the N content of the ith size fraction (g kg-1). and Wi is the proportion of the ith size fraction in total soil (%). Owing to the coupled relationship between soil C and N (Lan et al. 2020, Zhao et al. 2021), we suggested that soil N changed was similar to soil C following land use change and applied the procedure recommended by Wei et al. (2013) and Zhong et al. (2019) to evaluate the relative contribution of changes in aggregate mass and aggregated-associated N contents to the total changes in N stocks within each aggregate fraction. We assumed that changes in N stock within any particular aggregate fraction were induced both changes in N content of aggregate fraction (F1) and changes in the mass of aggregate fraction (F2). Hence, the F1 and F2 were calculated using the following equations:
Where F1 is the change in N stock (Mg N hm-2) aggregate associated due to the changes in N content aggregate-associated, F2 is the change in N stock within an aggregate fraction due to the changes in the mass of aggregate fractions, M is the initial mass of the aggregate fraction in MS, ΔM is the change in the mass of a particular fraction after cropland conversion, N is the final N content of the aggregate fraction (g kg-1) after conversion, and ΔN is the change in the N content of the aggregate fraction (g kg-1) caused by cropland conversion.
Statistical data analysis
A two-way analysis of variance (ANOVA) was conducted using SPSS 21.0 (SPSS, Inc., Chicago, IL, USA) to test the effects of cropland use types and soil depth on (i) bulk soil N content and stock, (ii) the content and stock of N aggregate-associated and (iii) C:N ratio in bulk soil and associated aggregate fractions. Pearson’s correlation analysis was performed to identify the relationships between N stocks in bulk soil and the proportion of aggregate and its N content and stock. The difference was considered significant if the p value was less than 0.05.
Soil N content and stock in bulk soil
According to two-way ANOVA, soil N content and stock were significantly affected by cropland use type (Table 1, p < 0.01). Soil N content and stock in FG field was significantly higher than that in other three cropland use types (Fig 1, 2, p < 0.05). However, there was no significant difference in soil N content and stock among MS, SG and MB fields (Fig 1, 2). Compared with MS, soil N content increased 54.37%, 50.37% and 30.00%, soil N stock increased 47.26%, 57.06% and 25.40% in FG field at 0–10, 10–20 and 20–30 cm, respectively (Fig 1, 2). Moreover, soil N content and stock significant decreased with soil depth (Table 1, p < 0.05).
Aggregate distribution and its N content
Among the five aggregate fractions, macro-aggregates (>2 mm) dominated the majority portion (77.14 ± 1.4%, averagely) of total soil, whereas micro-aggregates (<0.25 mm) accounted the lowest proportion (4.6 ± 0.52 %, averagely) (Table 3). Cropland conversion significantly impacted the amount of meso-aggregates (0.5–0.25 mm) and micro-aggregates (<0.25 mm), which was significant higher in MB, followed by SG, and lower in MS and FG (Table 1, 3, p < 0.05). Furthermore, the proportion of macro-aggregates was significantly increased while that of meso-aggregates (0.5–0.25 mm) and micro-aggregates decreased when MS converted to FG overall (Table 3), and no significant differences in the proportion of above mentioned fractions were observed in SG and MB soils (Table 3).
Soil N content and stock in bulk soil and associated-aggregate was significantly affected by cropland use types with exception in the size of 0.5–0.25 mm size fraction. They were significant higher in FG field than other three cropland use types and there was no significant difference in N content and stock in MS, SG and MB field (Fig 2d). Soil N content and stock in bulk soil and aggregate fraction presented obvious decrease trends with soil depth (Table 1). The increment in N stock associated with macro-aggregate was higher than that with meso-aggregate and micro-aggregate (Table 4), and the highest increment was observed in FG field (increased by 3.72, 2.98 and 3.24 kg N hm-2 at 0–10 cm, 10–20 cm and 20–30 cm compared with MS, respectively) (Table S2). The distribution of N stock in aggregate fractions was varied with aggregate sizes. Overall, soil N stock in macro-aggregate was accounted majority of total N stock (70.42 ± 3.89% at 0–10 cm, 72.67 ± 3.18% at 10–20 cm and 77.73 ± 3.02% at 20–30 cm, respectively) (Fig 3), followed by meso-aggregate (20.31 ± 2.42% at 0–10 cm, 19.24 ± 2.23% at 10–20 cm and 16.27 ± 2.08% at 20–30 cm, respectively) and micro-aggregate (9.27 ± 1.79% at 0–10 cm, 8.10 ± 1.55% at 10–20 cm and 6.00 ± 1.50% at 20–30 cm, respectively) (Fig 3).
C:N ratio in bulk soil and aggregate fractions
The ratio of C:N in bulk soil, ranged from 6.36 to 13.67, which was not significantly affected by cropland use types and soil depth (Table 1, p> 0.05). However, the significant effect of cropland use type on C:N aggregate–associated was observed in macro-aggregate (>2 mm), micro-aggregate (<0.25 mm) and 2–1 mm size class fraction, which significant higher in SG field and lower in FG field (Table 3). Furthermore, these variations were particular happened at 0–10 cm soil layer (Table 3). Soil C:N in bulk soil was narrowed changed with soil depth while C:N within aggregate was significantly decreased with soil depth expect within micro-aggregate (Table 1).
Contribution of the changes in aggregate dynamics to soil N accumulation
Based on the relationship between soil N stock in bulk soil and the mass and N aggregate-associated, we found that the soil N stock of bulk soil was significantly and positively correlated with N content and stock associated-aggregates (Fig 4, p< 0.05). However, there existed a significant negative relationship between soil N stock in bulk soil and the amount of micro-aggregate (Fig 4, p< 0.05). Notably, the slope and R2 of the linear relationships between N stock in bulk soil and N content in 0.5–0.25 mm, micro-aggregate and N stock in macro-aggregate (R2=0.5–0.58) was higher than those between N stock of bulk soil and indicators of aggregate dynamics in other size class (R2= 0.19–0.46) (Fig 4). In accordance with the results of linear regression analysis, the changes of bulk soil N stock after cropland conversion is mainly attribute to the changes of soil N stock associated with macro-aggregate across 0–30 cm soil layer, followed by the N stock associated with 2–1 mm size fraction(Table 4). Differently, the increase of N stock in bulk soil after MS converted to MB is primarily attributed to the N stock associated with micro-aggregate at 0–10 cm soil layer (Table 4). Moreover, the value of F1 in FG was visibly higher than that in SG and MB field, regardless of soil depth and aggregate size class (Fig 5).
Soil N content and stock in bulk soil
Our results showed that FG cultivation has a greater capacity for soil N stock than MS cultivation in karst region (Li et al. 2018a)，which was highly in line with previous research. The higher soil N stock in FG was mostly likely due to its higher root residue inputs and reduced tillage, which thus contributed to the increase in soil N pool. For instance, the biomass of fine root and its C and N storage was comparatively higher in FG than in MS field (Table S1). In addition, soil N availability improved based on its higher nitrate N and gross N mineralization rate in FG soil (Li et al. 2021). Moreover, the properties of N-fixing bacteria in FG may be another reason that N can fixed in the soil from the atmosphere in the form of root residues. For detail, a higher soil N content and a lower C:N ratio in fine roots was found at FG field (Table S1), which is conductive to the rapid release of N from plant tissue into the soil. Furthermore, Liang et al. (2016) and Xiao et al. (2020) demonstrated that forage grass cultivation promoted the diversity of N-fixing microorganisms to form root nodules for symbiotic N-fixation, and thus leaded to an increase in soil. Overall, FG cultivation has a higher N accumulation capacity than MS cultivation in karst region, southwest China.
Soil aggregate distribution and associated N pool
In this study, macro-aggregate (>2 mm) was the dominant aggregate size among four cropland use types, which agree with other studies (Xiao et al. 2021). However, Dou et al. (2020) and Wang et al. (2020) demonstrated that micro-aggregate (< 0.25 mm) occupied the mainly structure (40–89%) of soil across 0–30 cm following vegetation restoration. Compared with MS, the proportion of macro-aggregate was increased in FG to some extent (Table 3), indicating that MS converted to FG could facilitated the formation of macro-aggregate and then improved soil structure. In agriculture system, frequent tillage disruption can mechanically shock the aggregates and bring subsoil to the surface, which can readily promote soil OM turnover and hinder macro-aggregate formation (Hontoria et al. 2016, Xiao et al. 2017, Weidhuner et al. 2021). Inversely, fewer tillage disturbances occurred in FG with relative much crop residues accumulate on the surface soil, not only resulting in an increase of SOM (Li et al. 2018a), but also led to an increase of soil fungi microbe, which stimulate the formation of aggregate through by promoting the greater enmeshment of soil particles by fungal hyphae (Li et al. 2018b, Lan et al. 2020, Xiao et al. 2020). Thus, soil with fewer tillage disturbances is easy to recovery physically, which improves the re-aggregation around SOM. In addition, fresh organic matter accumulates with cropland restoration and then acts as the main binding agent in the process of macro-aggregate formation (Wiesmeier et al. 2012). Moreover, the organic mucilages release rapidly at lower C:N ratio in plant residues and then stimulate the formation of soil aggregation (Zhong et al. 2019). In our study, the higher macro-aggregate amounts was observed in FG with a lower C:N ratio of root (Table S1). Overall, smaller tillage disturbance and higher inputs of roots resulted in the formation of macro-aggregate after cropland restoration. In fact, forage grass cultivation enhanced the reformation of soil macro-aggregate has been reported in (Xiao et al. 2021), which probably owing to its higher inputs of litter and roots and moss crusts development following cropland restoration.
The sensitivity of aggregate associated-N to cropland use change may affect its contribution to the N stock change in bulk soil (Gelaw et al. 2015, Zhong et al. 2021). In our study, soil macro-aggregate fraction is the most essential contributor to the increase of soil N stock, corresponding to the increase of N stock in macro-aggregate after cropland restoration (Fig 2). Here, we found that the percentage of N stock associated with macro-aggregate contribute to the increase of total soil N stock was more than 80% (Table 4), suggesting that soil N stock could be directly increased under macro-aggregate with higher N stock. Previous studies also confirmed that macro-aggregate prefer more N stock than other relative smaller size fractions in karst region (Xiao et al. 2017, Lan et al. 2020). Cropland restoration increases the amount of large aggregates, thus accelerating the incorporation of C and N associated with small aggregate fractions into large fractions and thus providing physical protection for them (Lan et al. 2020). Soil N content and stock associated with aggregate was also differed differently across 0–30 cm soil layer. There was a distinct decrease pattern of soil N content and stock in micro-aggregate. However, soil N content and stock in macro-aggregate and meso-aggregate (expect 0.5–0.25 mm size fraction) was narrowed changed with soil depth. One explanation is that the litter and root residues inputs were mainly happened in 0–10 cm soil layer, which was more facilitated to soil N accumulation (Du et al. 2019).
Owing to the tight coupled relationship between soil C and N (Lan et al. 2020，Zhao et al. 2021), we assumed that the change of soil N stock within aggregates is mainly attributed to two factors: (1) changes in the mass of special aggregate fractions and (2) changes in the N content associated-aggregate fractions (Wei et al. 2013, Zhong et al. 2019). Thus, a better understanding of the dynamic of mass and associated-N content within special soil aggregate is imperative for assessing the soil N sequestration potential following cropland restoration. We observed that the contribution pattern of aggregate mass and aggregate-associated N content to the net accumulation of soil N stock in aggregates change differently after cropland conversion. Our calculations showed that the increase in soil N stock within macro-aggregate fractions was mainly determined by the increase in the N content associated with macro-aggregate rather than by the changes in macro-aggregate mass, and corresponded to a significant correlation between bulk soil N stock and N content associated with macro-aggregate (Table 4, Fig 5). This results were consistent with the results observed by Lan et al. (2020), who reported that the increases in N stocks in large aggregates mainly contributed to the increase of N stocks in bulk soil. However, these patterns were occurred in FG soil. For SG and MB, fewer or less changes in the net N stock change occurred in SG and MB, so the contribution of the mass and aggregate associated N content and stock to net soil N stock change was very small.
Soil C:N ratio in bulk soil and aggregate associated
The ratio of C:N in soil was an important factor for determining the degree of soil organic matter decomposition, which is essential for soil C and N accumulation and soil aggregation (von Lützow et al. 2006). In our study, the ratio of C:N in bulk soil was ranged from 8.93 to 10.60 averagely among four cropland use types, which was lower than the national and global average levels (12.3 averagely) (Tian et al. 2010). This indicated that the soil in karst region may have N limitation and suggest some N fertilizer were applied at the growth period of crops. The ratio of C:N in bulk soil did not differ significantly among four cropland uses (Table 4), indicating that the rate of decomposition of organic C in bulk soil was similar. C and N are structural components, and their accumulation and consumption are relatively fixed, and their responses to environmental changes are similar (Cleveland and Liptzin, 2007). Xu et al. (2016) and Zhang et al. (2019) reported that soil C and N changed proportionately, which thus resulted in no net change in soil C:N ratio after farmland conversion or afforestation. Nevertheless, in our study, the ratio of C:N in aggregates was varied differently following cropland conversion. Overall, the ratio of C:N had a general downward trend with increased soil aggregate size, indicating that the mineralization rate in large aggregate was more quick than in small aggregate and then release more organic N in large aggregate. Thus, soil N accumulation was highly consisted with large aggregates.
Our study found that soil N in bulk soil and various aggregate fractions were altered significantly following cropland conversion. Forage grass cultivation enhanced the N content and stock in bulk soil and aggregate-associated, especial for macro-aggregate. The increase of N stock was attributed to the increase of N stock of macro-aggregate. The increase of macro-aggregate-associated N stocks was mainly depended on the increase of macro-aggregate-associated N content. Soil C:N ratio in macro-aggregate, 2–1 mm and micro-aggregate but not in bulk soil was significantly changed following cropland conversion. Overall, forage grass cultivation replaced maize-soybean cultivation was suggested as an ecological restoration model to enhance soil N sequestration potential, owing to its role in increasing N stock of aggregate in the karst degraded region of Southwest China.
Acknowledgements
This study was financially supported by the National Natural Foundation of China (31901129).
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Author Contribution
All authors contributed to the study conception and design. Li Wen, Dejun Li and Kongcao Xiao set up and conduct the experiment, Haiming Tang, Xiaoping Xiao and Chao Li analysis the data, Li Wen wrote this manuscript, Haiming Tang revised the manuscript. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Competing Interests
The authors declare that they have no known competing interests.
Additional information
Correspondence and requests for materials should be addressed to Li Wen.
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Table 1 The two-way ANOVA results for all soil variables
|
Cropland use type |
Soil depth |
Cropland use type × Soil depth |
||||
|
Soil variables |
F |
p |
F |
p |
F |
p |
|
Aggregate size distribution |
|
|||||
|
>2 mm |
2.34 |
0.09 |
0.92 |
0.41 |
0.41 |
0.86 |
|
2–1 mm |
0.21 |
0.89 |
0.74 |
0.48 |
0.46 |
0.83 |
|
1–0.5 mm |
0.98 |
0.41 |
0.80 |
0.46 |
0.41 |
0.87 |
|
0.5–0.25 mm |
2.91 |
0.05 |
0.53 |
0.59 |
0.26 |
0.95 |
|
<0.25 mm |
6.02 |
<0.01 |
0.47 |
0.63 |
0.17 |
0.98 |
|
N content |
||||||
|
Bulk soil |
6.55 |
<0.01 |
8.66 |
<0.01 |
0.37 |
0.89 |
|
>2 mm |
16.21 |
<0.01 |
0.36 |
0.70 |
0.10 |
1.00 |
|
2–1 mm |
17.12 |
<0.01 |
0.78 |
0.47 |
0.14 |
0.99 |
|
1–0.5 mm |
16.22 |
<0.01 |
1.00 |
0.38 |
0.08 |
1.00 |
|
0.5–0.25 mm |
21.41 |
<0.01 |
3.27 |
0.05 |
0.44 |
0.85 |
|
<0.25 mm |
22.14 |
<0.01 |
3.82 |
<0.01 |
0.30 |
0.93 |
|
N stock |
|
|||||
|
Bulk soil |
10.49 |
<0.01 |
10.29 |
<0.01 |
0.26 |
0.95 |
|
>2 mm |
21.03 |
<0.01 |
1.97 |
0.16 |
0.16 |
0.99 |
|
2–1 mm |
10.39 |
<0.01 |
1.29 |
0.29 |
0.06 |
1.00 |
|
1–0.5 mm |
4.13 |
<0.01 |
2.87 |
0.07 |
0.20 |
0.97 |
|
0.5–0.25 mm |
2.11 |
0.12 |
6.00 |
<0.01 |
0.27 |
0.95 |
|
<0.25 mm |
3.84 |
<0.05 |
11.13 |
<0.01 |
0.37 |
0.90 |
|
C:N ratio |
||||||
|
Bulk soil |
1.95 |
0.14 |
0.56 |
0.58 |
0.36 |
0.90 |
|
>2 mm |
3.69 |
<0.05 |
8.03 |
<0.01 |
0.06 |
1.00 |
|
2–1 mm |
4.52 |
<0.01 |
6.18 |
<0.01 |
0.21 |
0.97 |
|
1–0.5 mm |
2.25 |
0.10 |
7.01 |
<0.01 |
0.33 |
0.92 |
|
0.5–0.25 mm |
2.61 |
0.07 |
5.31 |
<0.01 |
0.29 |
0.94 |
|
<0.25 mm |
5.95 |
<0.01 |
2.87 |
0.07 |
0.31 |
0.93 |
Bold values were significant at p < 0.05.
Table 2 Distribution of aggregate fraction (%) under four cropland use types
|
Size aggregate |
|||||
|
>2 mm |
2–1 mm |
1–0.5 mm |
0.5–0.25 mm |
<0.25 mm |
|
|
0–10 cm |
|||||
|
MS |
76.50 (8.60) |
12.13 (2.99) |
3.63 (1.65) |
2.93 (1.66) |
4.80 (2.77) |
|
SG |
68.73 (7.80) |
12.20 (2.66) |
5.03 (1.81) |
4.85 (1.82) |
9.18 (1.67) |
|
MB |
66.95 (7.73) |
10.25 (2.54) |
3.93 (1.17) |
5.55 (2.21) |
13.30 (4.75) |
|
FG |
82.63 (2.62) |
9.50 (1.59) |
2.18 (0.59) |
1.90 (0.63) |
3.80 (0.77) |
|
10–20 cm |
|||||
|
MS |
81.08 (5.19) |
9.93 (1.85) |
2.63 (0.96) |
2.18 (0.93) |
4.15 (1.50)b |
|
SG |
75.43 (6.70) |
10.68 (2.99) |
3.70 (1.33) |
3.40 (1.24) |
6.80 (1.66)b |
|
MB |
64.10 (3.73) |
11.10 (1.48) |
4.33 (0.90) |
6.10 (2.37) |
14.40 (4.16)a |
|
FG |
79.53 (5.28) |
11.97 (3.16) |
2.70 (1.08) |
1.90 (0.72) |
3.87 (1.25)b |
|
20–30 cm |
|||||
|
MS |
77.73 (4.73) |
11.20 (1.57) |
3.33 (0.86) |
2.95 (0.96) |
4.73 (1.52) |
|
SG |
81.55 (3.10) |
7.83 (0.90) |
2.48 (0.30) |
2.45 (0.23) |
5.70 (1.20) |
|
MB |
74.83 (8.15) |
7.65 (0.78) |
2.95 (0.90) |
4.13 (2.04) |
10.45 (5.55) |
|
FG |
83.27 (2.59) |
10.40 (3.39) |
2.07 (0.27) |
1.50 (0.30) |
2.77 (1.08) |
|
0–30 cm |
|||||
|
MS |
78.43 (3.40) |
11.08 (1.19) |
3.19 (0.64) |
2.68 (0.65)b |
4.56 1.06)b |
|
SG |
75.23 (3.54) |
10.23(1.35) |
3.73 (0.75) |
3.57 (0.73)ab |
7.23 (0.91)b |
|
MB |
69.04 (4.07) |
9.54 (1.05) |
3.68 (0.56) |
5.18 (1.17)a |
12.56 (2.65)a |
|
FG |
81.89 (1.87) |
10.51 (1.37) |
2.30 (0.37) |
1.78 (0.31)b |
3.51 (0.54)b |
Data are presented by means with standard error. Different letters indicate significant difference among four cropland use types (p < 0.05). MS: maize-soybean, SG: sugarcane, MB: mulberry, FG: forage grass.
Table 3 C:N ratio in bulk soil and aggregate fractions at 0–30 cm depths of various cropland use types.
|
Aggregate sizes |
||||||
|
Cropland use type |
Bulk soil |
>2 mm |
2–1 mm |
1–0.5 mm |
0.5–0.25 mm |
<0.25 mm |
|
0–10 cm |
||||||
|
MS |
9.95 (0.93) |
8.40 (0.51)ab |
8.80 (0.59)ab |
9.00 (0.57) |
9.91 (0.90) |
10.06 (0.76)a |
|
SG |
10.12 (1.37) |
9.33 (0.43)a |
9.49 (0.56)a |
9.28 (0.48) |
10.07 (0.82) |
10.33 (0.21)a |
|
MB |
9.46 (0.52) |
7.77 (0.33)ab |
7.76 (0.13)ab |
8.11 (0.14) |
7.78 (0.35) |
8.44 (0.08)ab |
|
FG |
9.57 (0.61) |
7.00 (0.99)b |
6.67 (1.08)b |
7.33 (1.45) |
7.09 (1.72) |
7.09 (1.31)b |
|
10–20 cm |
||||||
|
MS |
9.81 (1.10) |
7.25 (0.43) |
7.64 (0.73) |
7.15 (0.21) |
8.05 (0.17) |
8.64 (0.38) |
|
SG |
11.56 (0.89) |
8.13 (1.02) |
8.63 (1.08) |
8.25 (0.95) |
8.61 (0.75) |
8.98 (0.74) |
|
MB |
9.03 (0.82) |
6.54 (0.89) |
6.53 (0.69) |
7.04 (0.54) |
7.54 (0.98) |
7.39 (0.87) |
|
FG |
9.37 (0.48) |
5.83 (1.53) |
5.61 (1.28) |
5.96 (1.69) |
6.16 (1.50) |
6.40 (1.60) |
|
20–30 cm |
||||||
|
MS |
8.85 (1.15) |
6.29 (0.51) |
6.51 (0.63) |
7.09 (0.50) |
6.52 (0.66) |
7.46 (0.49)ab |
|
SG |
10.12 (0.71) |
6.71 (0.18) |
6.67 (0.59） |
6.22 (0.51) |
6.92 (1.50) |
9.47 (1.31)a |
|
MB |
8.30 (1.13) |
5.56 (0.73) |
6.02 (0.90) |
6.30 (0.44) |
6.13 (0.99) |
6.83 (0.68)ab |
|
FG |
8.92 (0.53) |
4.53 (1.27) |
5.18 (1.10) |
4.73 (1.35) |
5.70 (0.88) |
6.07 (1.20)b |
|
0–30 cm |
||||||
|
MS |
9.54 (0.57) |
7.31 (0.36)ab |
7.65 (0.44)a |
7.75 (0.36) |
8.16 (0.54) |
8.72 (0.44)a |
|
SG |
10.60 (0.57) |
8.05 (0.47)a |
8.26 (0.54)a |
7.92 (0.52) |
8.53 (0.68) |
9.59 (0.49)a |
|
MB |
8.93 (0.47) |
6.63 (0.45)ab |
6.79 (0.42)ab |
7.16 (0.32) |
7.11 (0.48) |
7.57 (0.38)ab |
|
FG |
9.28 (0.30) |
5.78 (0.72)b |
5.84 (0.63)b |
6.01 (0.84) |
6.33 (0.75) |
6.53 (0.71)b |
Data are presented by means with standard error. Different letters indicate significant difference among cropland use types (p < 0.05). MS: maize-soybean, SG: sugarcane, MB: mulberry, FG: forage grass.
Table 4 Effect of cropland use change in bulk soil N stocks and the contributions aggregate-associated N stock to bulk soil N stock
|
Changes in N stock in bulk soil |
Contribution (%) of change in aggregate-associated N stock to soil N stock in bulk soil |
||||||
|
Cropland conversion |
kg N hm-2 |
% change |
>2 mm |
2–1 mm |
1–0.5 mm |
0.5–0.25 mm |
<0.25 mm |
|
0–10 cm |
|||||||
|
–SG |
0.08 |
4.87 |
96.64 |
20.81 |
-1.34 |
-1.34 |
-14.77 |
|
–MB |
0.28 |
17.27 |
-43.64 |
-41.82 |
7.27 |
50.91 |
127.27 |
|
–FG |
0.75 |
47.25 |
82.25 |
8.96 |
2.05 |
2.16 |
4.59 |
|
10–20 cm |
|||||||
|
–SG |
-0.09 |
-6.49 |
48.39 |
3.23 |
8.06 |
14.52 |
25.81 |
|
–MB |
0.08 |
6.11 |
154.61 |
4.56 |
-3.17 |
-15.85 |
-40.15 |
|
–FG |
0.75 |
57.06 |
75.87 |
13.62 |
4.17 |
2.73 |
3.6 |
|
20–30 cm |
|||||||
|
–SG |
-0.23 |
-18.45 |
8.89 |
82.49 |
9.36 |
0.27 |
-1.01 |
|
–MB |
-0.03 |
-2.58 |
14.69 |
76.17 |
8.80 |
0.90 |
-0.55 |
|
–FG |
0.32 |
25.40 |
16.25 |
77.17 |
8.70 |
0.57 |
-2.69 |
|
0–30 cm |
|||||||
|
–SG |
-0.08 |
-5.77 |
90.27 |
31.28 |
-1.05 |
-2.52 |
-17.98 |
|
–MB |
0.11 |
7.73 |
240.43 |
67.57 |
-6.29 |
-50.14 |
-151.57 |
|
–FG |
0.61 |
43.65 |
80.47 |
11.22 |
2.70 |
2.00 |
3.62 |