Introduction
China was growing jasmine area in 1334 ha2 in 2015. China is one of the major producers of jasmine in the world accounting for about two-thirds of global planting area, and jasmine tea produced in Fuzhou accounts for more than half of Chinese production (Mei and Lin, Reference Mei and Lin2016). Fuzhou jasmine has the longest planting history, and it plays an important role in conserving water sources, protecting shores and promoting siltation, and ensuring biodiversity. In 2014, the Fuzhou Jasmine and Tea Culture System was listed as an important agricultural cultural heritage in the world, and its traditional jasmine tea tanning process was also rated as a national intangible cultural heritage (Min and Zhang, Reference Min and Zhang2015). Soil degradation is mainly manifested in the loss of soil organic matter (SOM), decline of fertility, element imbalance, deterioration of soil structure, and overall deterioration of soil environment (Jie et al., Reference Jie, Jing, Man and Zi2002; Selby, Reference Selby1993; Tamene et al., Reference Tamene, Sileshi, Ndengu, Mponela, Kihara, Sila and Tondoh2019). However, there are some problems of soil degradation such as changes in soil carbon (C), nitrogen (N), and phosphorus (P) contents and stoichiometries that still remain to be studied. Therefore, research on the soil C, N, P contents, storages, and ecological stoichiometry of jasmine croplands in Fuzhou under different degrees of degradation is of great significance for the future protection and sustainable use of agricultural cultural heritage.
C, N, and P play an important role in improving soil quality and maintaining terrestrial ecosystem productivity and vegetation growth (Gao et al., Reference Gao, He, Yu, Chen and Wang2014; Peñuelas et al., Reference Peñuelas, Poulter, Sardans, Ciais, van der Velde, Bopp, Boucher, Godderis, Hinsinger, Llusia, Nardin, Vicca, Obersteiner and Janssens2013, Reference Peñuelas, Janssens, Ciais, Obersteiner and Sardans2020; Sun et al., Reference Sun, Bhatti, Jassal, Chang, Arevalo, Black and Sidders2015; Wen et al., Reference Wen, Dong, Li, Li, Shi, Wang, Liu and Ma2013). Dynamic changes in soil C, N, and P storage and availability can directly affect soil fertility (Musinguzi et al., Reference Musinguzi, Ebanyat, Tenywa, Basamba, Tenywa and Mubiru2016), which in turn affects ecosystem stability and recovery (Xu et al., Reference Xu, Liu, Fang, Chen, Liu, Liu, Yuan and Wu2019; Zhao et al., Reference Zhao, Dong, Jiang, Liu, Ji, Li, Han and Sha2017). Previous studies have found that different management modes have significant effects on soil nutrient content and its stoichiometric ratio of jasmine (Wang et al., Reference Wang, Min, Sardans, Wang, Asensio, Bartrons and Peñuelas2016). However, how soil nutrient content and its stoichiometric ratio change under the background of different degrees of degradation requires further research.
The stoichiometrical relationships of C:N:P are key for plant–soil interactions and their associated links with nutrient cycling, limiting elements, and global climate change impacts (Elser et al., Reference Elser, Fagan, Denno, Dobberfuhl, Folarin, Huberty, Interlandi, Kilham, McCauley, Schulz, Siemann and Sterner2000; Mooshammer et al., Reference Mooshammer, Wanek, Hämmerle, Fuchslueger, Hofhansl, Knoltsch, Schnecker, Takriti, Watzka, Wild, Keiblinger, Zechmeister-Boltenstern and Richter2014; Peñuelas et al., Reference Peñuelas, Janssens, Ciais, Obersteiner and Sardans2020; Sardans and Peñuelas, Reference Sardans and Peñuelas2013; Tian et al., Reference Tian, Chen, Zhang, Melillo and Hall2010; Xia et al., Reference Xia, Yu, Wang and Xie2014; Zechmeister-Boltenstern et al., Reference Zechmeister-Boltenstern, Keiblinger, Mooshammer, Peñuelas, Richter, Sardans and Wanek2015). Current research on soil C, N, and P contents and storage has been mainly focused on different land types (Zhao et al., Reference Zhao, Zhang, Dong, Wu, Liu, Su, Wang, Zhang and Tang2018), different years (Muhammed et al., Reference Muhammed, Coleman, Wu, Bell, Davies, Quinton, Carnell, Tomlinson, Dore, Dragosits, Naden, Glendining, Tipping and Whitmore2018), and different management methods (Fahey et al., Reference Fahey, Woodbury, Battles, Goodale, Hamburg, Ollinger and Woodall2010; Knops and Tilman, Reference Knops and Tilman2000) but less on the effects of soil degradation in sites that used the same management mode and grow the same crop species.
The objectives of this study were to investigate: (1) soil C, N, and P contents and stocks and their stoichiometrical relationships under different degrees of soil physical degradation in jasmine croplands of Fuzhou; (2) the correlation of contents, stocks, and stoichiometries with soil factors (bulk density, pH, water content, salinity, clay, silt, and sand) under different degrees of degradation; and (3) the key physicochemical factors associated to soil degradation in the jasmine planting area. We also aimed to use this investigation as a tool to propose changes in cropland management.
Materials and Methods
Study sites
This study was located in the Fuzhou main jasmine planting area in southeastern China (118°45′13′′˜119°39′19′′ E; 25°36′24′′˜26°20′22′′ N) (Figure S1). The basic information of each sampling point is shown in Table S1. The main jasmine planting areas are located in the Minjiang River basin, which is a warm and humid subtropical monsoon climate with a frost-free period of 326 days and an average annual precipitation of 1349 mm (Wang et al., Reference Wang, Min, Sardans, Wang, Asensio, Bartrons and Peñuelas2016). The soil is mainly composed of slightly acidic red soil and is suitable for jasmine growth.
Sample collection and determination
To conduct this study, we first selected randomly 11 jasmine planting sites (one without soil degradation, five with low level of soil degradation, two with medium levels of soil degradation, and three with high level of soil degradation) in Fuzhou area. We based the selection on the principle of equal, random, and multipoint mixing. We classified these sites in different groups depending on their level of soil degradation following the classification method of Pieri (Pieri, Reference Pieri1992). The area (Figure S1) was sampled during the jasmine-growing season from February to July 2017. We took 5 different soil sample replicates in each one of the 11 jasmine-growing areas. To standardize the study, the jasmine genotype used in all the 11 jasmine-planting areas was the same one: double-jasmine genotype. Plant and row space is 30 × 35 cm.
Fertilization: Compound fertilizer (N-P2O5-K2O = 16–16–16) and urea (46% N) are the main fertilizer applied in this study. Every year, from February to March after cutting branches, urea is applied at a dosage of 4.2 t N km−2, whereas after flowering peak, compound fertilizer is applied at a dosage of 18 t km−2, 7.5 t K2O km−2, and 30 t P2O5 km−2.
Water management: As stated earlier, the jasmine in Fuzhou is mostly planted in riverside wetlands and shoals, and the altitude and topography are relatively low (Table S1). Therefore, jasmine basically does not need to be watered, but it is watered when it is particularly dry (Wang et al., Reference Wang, Min, Sardans, Wang, Asensio, Bartrons and Peñuelas2016). This mostly occurs in summer and autumn when watering is applied in the early morning.
Pruning: At the end of the annual flowering period, some branches are pruned. Due to the differences in the specific conditions of each sampling site (geographical location, altitude, small terrain, etc., Table S1), there are subtle differences in management measures (Table S1). Using a small core sampler (length and diameter of 0.5 and 0.1 m, respectively), the soil of the 0˜15 cm was collected and introduced into a ziplock bag, and immediately transported to the laboratory. After picking up the roots of the plant residue, each sample was divided into two parts. One part was placed in a refrigerator at 4 °C to be later used to measure soil pH, bulk density, and water content. The other part was air dried, put it into a ziplock bag, and kept until analysis of soil particle size, soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP).
After the samples were taken back to the laboratory, impurities (stones, dead leaves, etc.) in the soil were removed. (1) The samples were naturally dried indoors and ground through a 10-mesh sieve; (2) take a certain amount of samples, add 5% dilute hydrochloric acid to stir for many times, add dilute hydrochloric acid to the reaction is complete, and soak for 24 hr; and (3) wash with neutral deionized water until neutral (pH = 7). After drying, grind the sample to powder in an agate mortar and pass it through a 100-mesh sieve. SOC and TN were determined using a CN elemental analyzer (Elementar Vario MAX CN, Hanau, Germany), and TP was decongested by HCIO4–H2SO4 method in a continuous flow analyzer (San++, SKALAR Corporation production, Breda, Netherlands). SOC data were converted to SOM using the van Bemmelen factor of 1.724 (Heaton et al., Reference Heaton, Fullen and Bhattacharyya2016).
Other soil variables were also analyzed. Soil water content was determined by the drying method (Lu, Reference Lu1999), and bulk density was measured for the three bulked cores (5-cm diameter and 3-cm depth) collected from each soil layer. Soil salinity and pH were measured using a 2265FS salinity meter (Spectrum Technologies Inc., Paxinos, USA) and a handheld pH meter (STARTER 300, Parsippany, USA), respectively. Particle size (clay, silt, and sand) was measured by a Mastersizer 2000 laser particle-size analyzer (Malvern Scientific Instruments, Suffolk, UK). (1) An amount of 0.5 g of soil sample were weighed, then 30% hydrogen peroxide was added, organic matter was removed at 72 °C, and hydrochloric acid was added to remove carbonate. (2) Ultrapure water was added to dilute until pH was 6.5–7. (3) Then, sodium hexametaphosphate was added, ultrasonication was conducted for 30 s, and afterwards the laser particle-size analyzer Mastersizer 2000 measured the volume percentage of soil particle size.
Calculation of soil C, N, and P storages
The C, N, and P storages of the soil surface profile at a certain depth per unit area were calculated by the following equation:
$$\,{T_x} = \Sigma {dx \times {M_x}} \times d \times 10,$$
where x of T x , D x , and M x represents a substitution for C, N, or P. T x is the soil C (TC), T (TN), P (TP) storages (t km−2), D x is the soil surface bulk density (g cm−3), M x is the soil surface C, N, and P content (g kg−1), and d is the soil depth (cm).
Soil degradation index and threshold
The soil structural stability index (St) is an important indicator of soil structural degradation and an important indicator of SOM to maintain soil structural stability (Pieri, Reference Pieri1992). This indicator was calculated using the following equation:
$$St{\mkern 1mu} = {\mkern 1mu} 100\% {\mkern 1mu} \times {\mkern 1mu} {{{\rm{SOM}}\left( \% \right)} \over {{\rm{Clay}}\left( \% \right){\mkern 1mu} {\rm{ + }}{\mkern 1mu} {\rm{Silt}}\left( \% \right)}}.$$
In the formula, when St ≤ 5%, it indicates that the soil structure is degraded due to the large loss of SOM; when St > 5%, there is no risk of degradation of soil structure (Pieri, Reference Pieri1992). In this study, the St ≤ 5% (degraded structure) was further subdivided, when 4% ≤ St < 5%, the soil structure degradation was considered low degree; when 3% ≤ St < 4%, middle degree; when St < 3%, high degree. Therefore, the S1 site belonged to high-risk degradation but without current degradation; the S2, S3, S4, S5, and S6 sites to low degree of degradation; the S7 and S8 sites to middle degree of degradation; and the S9, S10, and S11 sites to high degree of degradation (Figure S2).
Statistical analyses
We conducted a one-way analysis of variance using SPSS 20.0 to analyze the data difference between among the degrees of degradation. The correlations between different degrees of degradation and soil properties were determined by Pearson correlation analysis. We also performed multivariate statistical analyses using functional discriminant analysis (FDA) to determine the overall differences of soil clay, silt and sand contents, total C, N, and P contents, soil C:N, C:P, and N:P ratios, soil bulk density, salinity, pH, and water content among the different levels of soil degradation. Discriminant analyses consist of a supervised statistical algorithm that derives an optimal separation between groups established a priori by maximizing between-group variance while minimizing within-group variance. FDA is thus an appropriate tool for identifying the variables most responsible for the differences among groups. The FDA was performed using Statistica 8.0 (StatSoft, Inc., Tulsa, USA).
Results
Analysis of soil C, N, and P contents in jasmine cropland under different degrees of degradation
Compared with nondegraded soil, the contents of degraded SOC showed a significant decrease trend (P < 0.05, Figure 1a). We have also observed that TN contents in the middle degree of degradation and high degree of degradation were 42.2% and 33.3% lower, respectively, than in the nondegraded soils (P < 0.05, Figure 1b). However, no differences in soil TP contents were observed among all treatments (P > 0.05, Figure 1c).
Figure 1. Soil C (a), N (b), and P (c) contents, soil C (d), N (e) and P (f) storages and soil C:N (g), C:P (h) and N:P (i) ratios in jasmine croplands of Fuzhou under different degrees of degradation (same for other captions). Different lowercase letters in the figure indicate significant differences between sampling points (P < 0.05). Error bars represent standard errors of the means (n = 5). HDD, high degree of degradation; LDD, low degree of degradation; MDD, middle degree of degradation; ND, no degradation.
Analysis of soil C, N, and P storages in jasmine cropland under different degrees of degradation
Compared with nondegraded soils, the soil C and N storages showed a significant decrease trend in the middle degree of degradation and the high degree of degradation (P < 0.05, Figure 1d, 1e). However, there were no differences in soil P storages among all treatments (P > 0.05, Figure 1f).
Characteristics and analysis of soil C, N, and P ecological stoichiometry in jasmine cropland of Fuzhou under different degrees of degradation
The soil C:N ratio was 18.5% higher in the middle degree of degradation than in the nondegraded soils (P < 0.05, Figure 1g), and compared with nondegraded soils, the degraded soils C:P and N:P ratios increased to a certain extent (P > 0.05, Figure 1h, 1i).
Regression analysis of soil C, N, and P contents in different jasmine planting areas in Fuzhou
C, N, and P contents were strongly correlated (P < 0.01, Figure 2a–2c). Along the studied sites, the relationship between C and N was stronger than the coupling relationships between C and P and between N and P.
Figure 2. Regression analysis of soil C (a), N (b), and P (c) contents in jasmine croplands of Fuzhou under different degrees of degradation.
Analysis of soil properties in jasmine cropland of Fuzhou under different degrees of degradation
Compared with nondegraded soils, the degraded soils bulk density showed a significant increase trend, but water content showed a significant decrease trend (P < 0.05, Figure 3a, 3b). The soil pH in the middle degree of degradation was 9.18% higher than in the nondegraded soils (P < 0.05, Figure 3c). The soil clay and silt in the high degree of degradation were 33.1% and 25.3% higher, respectively, than in the nondegraded soils (P < 0.05, Figure 3e). The soil sand in the high degree of degradation was 32.6% lower than in the nondegraded soils (P < 0.05, Figure 3g).
Figure 3. Soil bulk density (a), water content (b), pH (c), salinity (d), and clay (e), silt (f) and sand (g) concentrations in jasmine croplands of Fuzhou under different degrees of degradation (ND, LDD, MDD, and HDD). Different lowercase letters in the figure indicate significant differences between sampling points (P < 0.05). Error bars represent standard errors of the means (n = 5).
Correlation analysis between different soil degradation degrees and soil variables
The soil structural stability index was positively correlated with SOC, TN, TC, water content, and sand (P < 0.01), and negatively correlated with bulk density and silt (P < 0.01) and clay (P < 0.05) (Figure 4).
Figure 4. R values showing the Pearson correlations coefficients between soil degradation degrees and soil variables. The TN, TP, TC, TN, TP, BD, and WC represent total nitrogen content, total phosphorus content, total C storage, total N storage, total P storage, bulk density, and water content, respectively. Symbols * and ** indicate significant correlations at the 0.05, and 0.01 levels, respectively.
FDA
Soil pH was the main variable, followed by soil salinity, stability, and bulk density explaining the differences between soil samples of nondegraded soils and those of degraded soils (Figure 5, Table 1). Samples of middle- and high-degraded soils were not significantly different according with the FDA (Table 2).
Figure 5. Functional discriminant analysis (FDA) 2D layout (showing the plot formed by the scores and layouts of the two first roots, explaining a 98% of the total variance) with soil clay, silt and sand contents, total C, N, and P contents, soil C:N, C:,P and N:P ratios, soil bulk density, salinity, pH, and water content as independent continuous variables and different levels of soil degradation as categorical dependent variables.
Table 1. Statistical significance of the independent variables in the functional discriminant analysis with the soils with different level of degradation as the dependent categorical grouping variable
Bold type indicates significant differences (P old type TN, TP, and WC represent total nitrogen content, total phosphorus content, and water content, respectively.
Table 2. Test statistics for squared Mahalanobis distances among the soil groups with different level of degradation with soil clay, silt, and sand contents, total C, N, and P contents, soil C:N, C:P, and N:P ratios, soil bulk density, salinity, pH, and water content as independent continuous variable
Bold type indicates a significant effect of the variable in the model (P < 0.05).
Discussion
Effects of different soil degradation degrees on chemicophysical soil traits
The decrease of soil pH coupled with salinity (conductivity) and the loss of aggregate stability are the main variables that distinguished degraded from nondegraded soils. The shifts in soil physical properties, for example, in soil density and porosity, were associated to a loss of SOC as observed normally under soil degradation processes (Ola et al., Reference Ola, Schmidt and Lovelock2018). The current management based on a certain level of organic matter incorporation but with a constant cut of aboveground plants leaves bare soil vulnerable to be easily loss by runoff and with scarce natural incorporation of organic matter. It is thus inadequate in this most important growing area of this crop. It leads to loose soil structure, increased soil porosity and oxygen content, and enhanced aerobic microbial activity (Zhang et al., Reference Zhang, Xin, Zhu, Yang, Zhang, Ding, Mu and Shao2018), which accelerates the decomposition rate of microorganisms on C, N, and other nutrients, resulting in less accumulation of soil C and N. Moreover, the surface of bare soil under long-term jasmine cultivation should favor runoff of water leading to further loss of C and N in jasmine soil contributing to increase the level of soil degradation.
These results should be taken into account to shift the current management towards larger inputs of organic matter. It would be advisable to lead underground growth and cut it and spread for soil when the jasmine plant begins to build the flowers to avoid competing underground vegetation with jasmine plants in this moment, and at the same time also achieve higher organic matter inputs to soil.
Effects of different soil degradation degrees on C, N, and P contents and storages in jasmine cropland
The total amount of soil nutrient is a general reference to characterize the potential level of soil nutrient supply at medium and long term (Bai et al., Reference Bai, Conant, Zhou, Wang, Wang, Li and Zhang2016). In this study, the C, N, and P contents and storages were different in the distinct soil degradation degrees. Soil C and N contents and storages were lower in degraded than in nondegraded soils. Soil C and N are affected by many factors such as decomposition of soil parent material, litter, plant roots, and absorption and utilization of plants (Lane et al., Reference Lane, Noske and Sheridan2011). In general, higher rates of soil degradation mean lower productivity, usually as a decline in aboveground and below biomass production (Jiang et al., Reference Jiang, Han and Wu2006; Zhou et al., Reference Zhou, Li, Zhao and Drake2008). The deepening of soil degradation leads to limited vegetation growth, which reduces the content of litter, resulting in a decrease in C and N contents in the soil (Xu et al., Reference Xu, Liu, Fang, Chen, Liu, Liu, Yuan and Wu2019). Barren land together with monsoon climate with heavy rains results in a not-adequate combination to the accumulation of organic matter. The soil N input is mainly dependent on the return of plant residues, biological N fixation, and the input derived from the dry and wet deposition of the atmosphere (Abaker et al., Reference Abaker, Berninger, Saiz, Pumpanen and Starr2018). However, due to the influence of artificial picking and other factors, the amount of jasmine plant residues is small, and the hilly topography is not conducive to the accumulation of N, so that the observed C and N storages were lower. Other studies have also shown that higher soil structural degradation drives to lower SOC content (Tamene et al., Reference Tamene, Sileshi, Ndengu, Mponela, Kihara, Sila and Tondoh2019), consistent with our observations. This also should in great part be due to the intensive agricultural model of the jasmine crops, which mainly exports materials and energy, and the lost materials are not supplemented in time and effectively, resulting in continuous loss of SOC and lower stability of soil aggregates. In addition, the leaching of long-term rainwater easily leads to the loss of some surface organic matter. Previous studies have demonstrated that organic cultivation improves soil fertility and C accumulation in jasmine croplands of Fujian Province (Wang et al., Reference Wang, Min, Sardans, Wang, Asensio, Bartrons and Peñuelas2016). Therefore, organic fertilizer can be appropriately added to alleviate the lack of organic matter, and appropriate burial can be conducted to reduce nutrient loss.
However, there was no significant difference in soil P content under different degrees of degradation in this study, and the overall P contents (0.70 g kg−1) were lower than the average level of soil TP in China (0.975 g kg−1) (Tian et al., Reference Tian, Chen, Zhang, Melillo and Hall2010). These low contents of soil P also strongly suggest an inadequate management and fertilization strategies. Moreover, when the content of certain essential elements in the soil is low, the plants will maximize the uptake of this element to counteract the leaching of the nutrients in the soil and thus maintain the normal growth of the plant (Lambers et al., Reference Lambers, Raven, Shaver and Smith2008). Anyway, this observed general lower P soil content is also consistent with the geographical distribution pattern of soil TP content in low latitudes of Houlton and Tian et al. (Houlton et al., Reference Houlton, Wang, Vitousek and Field2008; Tian et al., Reference Tian, Chen, Zhang, Melillo and Hall2010). In this study, the overall soil P storages are relatively low, as common in subtropical regions (Chen et al., Reference Chen, Niklas, Liu, Fang, Wan and Wang2015). Although planting jasmine increases the input of P into the soil in litter, the available P from decomposition is easily reabsorbed by plants (Xu et al., Reference Xu, Liu, Fang, Chen, Liu, Liu, Yuan and Wu2019), resulting in relatively low levels of P and P storages in the soil.
Effects of different soil degradation degrees on soil stoichiometry in jasmine croplands
Soil C:N:P stoichiometry is an important soil trait related to basical soil functions such as mineralization capacity and soil microbial community structure (Drenovsky and Richards, Reference Drenovsky and Richards2006; Mooshammer et al., Reference Mooshammer, Wanek, Hämmerle, Fuchslueger, Hofhansl, Knoltsch, Schnecker, Takriti, Watzka, Wild, Keiblinger, Zechmeister-Boltenstern and Richter2014). When the soil C:N < 25, microbial activity is enhanced and the organic matter is easy to decompose (Mooshammer et al., Reference Mooshammer, Wanek, Hämmerle, Fuchslueger, Hofhansl, Knoltsch, Schnecker, Takriti, Watzka, Wild, Keiblinger, Zechmeister-Boltenstern and Richter2014). In this study, the soil C:N ratio is lower than 25 under different degrees of degradation, which together with the high temperatures and abundant precipitation allowing high microbial activity favoring faster rate of organic matter and lowering of the C:N ratio. The soil N:P ratio can often be used to characterize the limiting role of these nutrients (Elser et al., Reference Elser, Bracken, Cleland, Gruner, Harpole, Hillebrand, Ngai, Seabloom, Shurin and Smith2007; Peñuelas et al., Reference Peñuelas, Poulter, Sardans, Ciais, van der Velde, Bopp, Boucher, Godderis, Hinsinger, Llusia, Nardin, Vicca, Obersteiner and Janssens2013, Reference Peñuelas, Janssens, Ciais, Obersteiner and Sardans2020), and the balance between the availability of N and P in soil also generally determines the plant’s N:P ratio (Wassen et al., Reference Wassen, Olde Venterink and de Swart1995). The N:P ratio of the study (4.01) is lower than that of global grassland (12.3) and forests (14.6) (Cleveland and Liptzin, Reference Cleveland and Liptzin2007), indicating that N may be the main limiting nutrient in jasmine cropland soil, consistently with the observed in previous studies (Guo and Jiang, Reference Guo and Jiang2019; Wang et al., Reference Wang, Sardans, Wang, Zeng, Tong, Asensio and Peñuelas2015).
Environmental effects of the soil degradation
Soil degradation was accompanied by decreased soil water content, organic matter, N content and storage, whereas the bulk density increased. The reduction of soil C storage indicates an increase of the emission of carbon dioxide to the atmosphere and thus a decreased ability to mitigate greenhouse gases (Makhalanyane et al., Reference Makhalanyane, Valverde, Gunnigle, Frossard, Ramond and Cowan2015). The loss of vegetation cover and the subsequent loss of SOC are the root causes of soil degradation (FAO and ITPS, 2015).
The lower N:P ratio than in the global grasslands and woodlands (Cleveland and Liptzin, Reference Cleveland and Liptzin2007) indicates that there may be N limitation in the jasmine soil, and therefore soil degradation will further aggravate it. Weesies et al. (Reference Weesies, Livingston, Hosteter and Schertz1994) found that soil degradation reduced the SOM content of various Indiana soils by 16–39%, TP by 29–38%, and clay content by 11–53%, which in turn led to crop yield reduction. Organic cultivation improves soil fertility and C accumulation in jasmine croplands of Fujian Province (Wang et al., Reference Wang, Min, Sardans, Wang, Asensio, Bartrons and Peñuelas2016).Therefore, organic fertilizer can be appropriately added to alleviate the lack of organic matter, and appropriately buried to reduce nutrient loss. Moreover, steel slag and biochar also can decrease greenhouse gases emission and improve the soil acid problem and enhance nutrient availability in jasmine field (Jin et al., Reference Jin, Liu, Wang, Wang, Min, Wang, Sardans, Liu, Song, Huang and Peñuelas2020), so they can become effective methods to restore the degraded jasmine land.
Conclusions
Our research provides theoretical and practical guidance for clarifying the spatial distribution and driving factors of soil degradation in jasmine croplands in Fuzhou, China. The results show that the level of degradation in jasmine croplands in Fuzhou is considerable and it is characterized by soil acidification and soil compaction with higher clay content and bulk density. The process of physical degradation is accompanied by important decreases of soil C and N contents. The decrease of N content is especially critical for this N-limited region (Figure 6). Moreover, Fuzhou jasmine region is also P deficient, which is aggravated by degradation process. The jasmine soils in Fuzhou are being increasingly degraded and losing organic C. Thus, higher inputs of organic fertilizer should be considered to alleviate the lack of organic matter, and appropriate burial should be conducted to reduce nutrient loss, and to, at least partially, cover the bare soil. Moreover, a rise of N and P fertilizers is also advisable.
Figure 6. A conceptual diagram of C, N, and P content, storage, and stoichiometry in the process of soil degradation in jasmine croplands in subtropical China. The figure above shows the sampling points of jasmine and physical factors that we observed at different degrees of degradation. The brown triangle shows that as the soil gradually degrades, the soil water content, sand, SOC, TN, and TP all show a decrease trend. By contrast, the green triangle represents soil salinity, soil bulk density, silt, and N:P ratio increasing with soil degradation.
Acknowledgements
This work was financially supported by the National Science Foundation of China (41571287), the Research Project of Public Institute of Fujian Province (2018R1034-1), the Outstanding Youth Scientific Research Talents Cultivation Plan in Colleges and Universities of Fujian Province 2017, Science and Technology Program of Fuzhou City (2016-N-86; 2019-N-13). Gobally Important Agricultural Heritage Systems (GIAHS) Monitoring Research Project of Fuzhou Agriculture and Rural Bureau 2021–2023. The authors would like to thank Wenwen Yang, Yannan Wang, and Yan Huang for their assistance with field sampling. J.P. and J.S. acknowledge support from the European Research Council Synergy grant no. ERC-2013-SyG 610028-IMBALANCE-P. The authors declare no conflict of interest.
Supplementary Material
To view supplementary material for this article, please visit https://doi.org/10.1017/S0014479721000089
