Responses of soil specific enzyme activities to short-term land use conversions in a salt-affected region, northeastern China

Science of the Total Environment 687 (2019) 939–945

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Responses of soil specific enzyme activities to short-term land use conversions in a salt-affected region, northeastern China Pujia Yu a,b,c,d, Xuguang Tang a,c,d, Aichun Zhang e, Gaohua Fan b, Shiwei Liu a,b,c,d,⁎ a

Chongqing Jinfo Mountain Field Scientific Observation and Research Station for Karst Ecosystem, School of Geographical Sciences, Southwest University, Chongqing 400715, China Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changhcun 130102, Jilin, China Research Base of Karst Eco-environments at Nanchuan in Chongqing, Ministry of Nature Resources, School of Geographical Sciences, Southwest University, Chongqing 400715, China d State Cultivation Base of Eco-agriculture for Southwest Mountainous Land, Southwest University, Chongqing 400715, China e College of Mobile Telecommunications, Chongqing University of Posts and Telecom, Chongqing 401520, China b c

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The specific enzyme activities in 10–20 cm depths were more sensitive to land use conversions than that in 0–10 cm depth. • Soil specific enzyme activity is a more appropriate indicator for identifying changes following land use conversions. • Re-vegetation increased the specific enzyme activities in northeastern China

a r t i c l e

i n f o

Article history: Received 3 April 2019 Received in revised form 27 May 2019 Accepted 11 June 2019 Available online 12 June 2019 Editor: Xinbin Feng Keywords: Re-vegetation Solonetz Geometric mean Soil enzyme activity

a b s t r a c t Soil enzyme activity is a sensitive indicator of soil quality changes. The response of soil enzyme activity to different land uses is important in addressing the issues of agricultural sustainability. The objectives of this study were to investigate the effects of short-term land use conversions on soil specific enzyme activity (per unit microbial biomass carbon) of sodic soils and compare the responses of soil absolute (per unit soil mass) and specific enzyme activities in northeastern China. Four specific enzyme activities, including catalase, invertase, urease and alkaline phosphatase were assayed at 0 to 20 cm depth under five land uses, including cropland (CL), alfalfa perennial forage (AF), monoculture grassland (AG), monoculture grassland for hay once a year (AG M) and successional regrowth grassland (RG). The specific activities of catalase, urease and alkaline phosphatase at 10 to 20 cm depth were 117.3%, 40.0% and 35.6% higher than that in 0 to 10 cm depth, irrespective to the land uses. Conversion of cropland to revegetation land increased the specific activities of catalase (2.8%), invertase (99.0%), urease (14.3%) and alkaline phosphatase (14.0%). Under land uses of AF, AG M, AG and RG, the geometric mean (0.2%, 32.8%, 65.7% and 24.3%, respectively) and sum (2.6%, 38.0%, 82.8% and 29.6%, respectively) of specific enzyme activities at 0 to 20 cm depth were higher than that under CL treatment. The soil specific enzyme activities showed the better discrimination to different land uses than the soil absolute enzyme activities. In conclusion, re-vegetation has a positive effect on the improvement of soil enzyme activity in northeastern China, and the responses of soil specific enzyme activities to short-term land-use conversions are more obvious than the absolute enzyme activities, which could be used as s suitable and sensitive indicator of land use change in semiarid agroecosystems. © 2019 Elsevier B.V. All rights reserved.

⁎ Corresponding author at: 2 Tiansheng Road, Chongqing 400715, China. E-mail address: [email protected] (S. Liu).

https://doi.org/10.1016/j.scitotenv.2019.06.171 0048-9697/© 2019 Elsevier B.V. All rights reserved.

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1. Introduction Conversion of cropland to a plant covered land is commonly used as an efficient measure to repair degraded agricultural ecosystems worldwide (Deng et al., 2016; Novara et al., 2017), since it is accompanied by significant improvements of plant cover and community composition, net primary production, carbon sequestration and nutrient accumulation (Raiesi and Beheshti, 2014; Pandey et al., 2014; Romero-Diaz et al., 2017; Raiesi and Salek-Gilani, 2018). These changes in soil ecosystem processes, functions and services could be known by changes in most soil biochemical properties (Mayor et al., 2016; Fatemi et al., 2016; Xu et al., 2017). Soil enzyme activities are frequently used as basic and sensitive indicators of microbiological and biochemical processes because they could supply rapid and accurate information on slight variations in microbial community activity, soil organic matter and other soil properties result from the land use conversions or agricultural management practices (Torres et al., 2015; Hu et al., 2018; Sanchez-Hernandez et al., 2018; Nannipieri et al., 2018; Li et al., 2019). The activities of soil enzymes relate to carbon, nitrogen and phosphorus cycling in soils usually increase when the cropland is converted to grasslands because of the increases of microbial activity, soil organic matter and the improvement of soil habitat (An et al., 2009; Veres et al., 2015; Yu et al., 2017; Raiesi and SalekGilani, 2018). Wang et al. (2011) in the Loess Plateau of China found that the improvement of soil properties and aeration conditions in revegetation land increased the soil microbial biomass and diversity, and thus resulted in acceleration of soil enzymes activities. Zhang et al. (2012) in the south of Qinling Mountains reported that the suffice substrates produced by the established vegetation and their roots during secondary succession increased enzyme activity. Raiesi and SalekGilani (2018) in a semi-arid rangelands of Iran found that the increase in soil aggregate stability in the abandoned farmlands enhanced the enzyme protection through the entrapment and inclusion in microaggregates, and then led to the augment of the enzyme activities. Besides, previous studies indicate that the nutrient availability in soils could also affect the enzyme activity (Bowles et al., 2014; Xu et al., 2017; Zhao et al., 2018). Bowles et al. (2014) in Yolo County, California illustrated microbes regulated enzyme activity to improve the nutrient limitations. Zhao et al. (2018) in Loess Plateau reported that phosphatase enzymes activities were enhanced by microbes in afforested ecosystems to meet the P limitations in soils. The significant changes in the inputs and outputs of organic matter into soils result from the land use conversions could affect the soil nutrient contents and their stoichiometric ratios, thus influencing the enzyme activities (Li et al., 2018; Yu et al., 2019). Although significant changes in soil enzyme activities are observed following land use conversions in different studies, the magnitude of changes in soil enzyme activities after land use conversions is significant different depending on the plant community type, the time and space scale one considers, the climate and the initial conditions of soil environment (Wang et al., 2012b; Raiesi and Beheshti, 2014; Looby and Treseder, 2018; Zhang et al., 2019). The composition and biomass size of soil microbial community determine the potential of enzymes production, and thus directly or indirectly influence the activity of soil enzymes (Raiesi and Beheshti, 2014; Torres et al., 2015; Lopez-Aizpun et al., 2018). The significant correlation between the soil absolute enzyme activity (per unit soil mass) and the microbial biomass make it is impossible to discriminate whether the inconsistency in the soil absolute enzyme activities under different land uses or management practices is the actual difference in enzyme activities or is caused by the difference in microbial biomass (Trasar-Cepeda et al., 2008; Raiesi and Beheshti, 2014). The soil specific enzyme activities (per unit microbial biomass carbon) is a useful indication of enzyme efficiency and their changes could be reflected the shifts in enzyme production and soil microbial community composition or a decline in the sorption of enzymes to mineral and the inclusion within stable aggregates or other primary factors that could be influenced soil enzyme

activities (Raiesi and Salek-Gilani, 2018). Using the soil specific enzyme activities to remove their obvious relevance with soil microbial biomass can help us to clearly understand the effects of land use conversions and obtain important ecological information from the changes of soil enzymes (Nsabimana et al., 2004; Liu et al., 2017; Raiesi and SalekGilani, 2018). However, few literatures are available on how soil specific enzyme activities is influenced by land use conversions (Liu et al., 2017; Raiesi and Salek-Gilani, 2018; Sliva et al., 2019). Due to the land degradation and corn yield oversupply, a series of policies and subsidies are issued by the Chinese government to help farmers to convert the corn cropland with unfavorable conditions for agricultural activity to forage or grasslands in northeastern China (Yu et al., 2017 and 2018). Changes of soil absolute and specific enzyme activity under different soil management practices are not consistent. Raiesi and Salek-Gilani (2018) in Iran found that the change trend of soil absolute and specific enzyme activities was contradictory, while similar change trend of soil absolute and specific enzyme activity to fertilizer application was found by Zhang et al. (2015) in southern China. The large and different responses of soil absolute and specific enzyme activities to land use conversions suggest more studies are needed to address these issues in different regions. Our previous study in the same site showed that the soil absolute enzyme activities were significantly positive correlated with the soil microbial biomass carbon and the conversion of cropland to grasslands significantly increased the soil absolute enzyme activities (Yu et al., 2017). However, whether the responses of soil specific enzyme activities to land-use conversions are similar to the absolute enzyme activities remains unknown. In this study, we hypothesized that the short-term re-vegetation could increase the specific soil enzyme activities in the salt-affected region of northeastern China, and the specific enzyme activity was more suitable than the absolute enzyme activity to show the responses of soil enzymes to land use conversions. To address this hypothesis, the main objectives of this research were to (1) investigate the responses in soil specific enzyme activities of alkaline soils to the studied land-use treatments, and (2) compare the changes of soil specific enzyme activity and absolute enzyme activity after land use conversions in northeastern China. 2. Materials and methods 2.1. Study area The study area is located in the southern Songnen Plain (44°33′ N and 123°31′ E, 145 m a.s.l.) in northeastern China. This area has a temperate, semiarid continental monsoon climate with long cold winter. The average annual temperature and precipitation between 1980 and 2013 is 5.9 °C and 427 mm, respectively. Distribution of annual precipitation is inhomogeneous and approximately 70%–80% of total precipitation occurs between June and September. The soil in the study area is Solonetz according to the World Reference Base for Soil Resources (IUSS Working Group, 2015). The land uses in the study area are covered by mosaic of corn cropland in the areas with high quality soils and native grasslands consisting of four dominant native species (i.e., Leymus chinensis (Trin.) Tzvelev, Chloris virgata Swartz, Puccinellia tenuiflora (Griseb.) Scribn and Elymus dahuricus Turcz) in the areas with low quality soils (Yu et al., 2018). 2.2. Experimental design The experiment, established in early May 2011 at a cropland, consisted of five land use treatments, and was carried out in a completed block design with four replications. The five land use treatments were as follows: (1) cropland (CL), which was under continuous corn monoculture since 2011; (2) alfalfa (Medicago sativa L.) perennial forage (AF), which was set up in May 2014. Before 2014, these treatments were croplands without tillage (no tillage) while the other practices

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were the same as those of the traditional practices; (3) successional regrowth grassland (RG), which was abandoned in 2011 to restore grassland without any disturbance; (4) monoculture Leymus chinensis (Trin.) Tzvelev grassland (AG); and (5) monoculture Leymus chinensis (Trin.) Tzvelev grassland for hay once a year (AG M). Seeds of Lyemus chinensis were sowed in May 2011 with a density of approximately 2000 seeds m−2 in the treatments of AG and AG M. The aboveground biomass reached approximately 120 g m−2 in the September 2011. The aboveground biomass under the AG treatment was returned into the soils as litter, and the vegetation under the AG M treatment was collected for hay at the middle of August once a year. The plot size was 12 m × 50 m for land use of CL and AF and 6 m × 50 m for land use of AG, AG M and RG. The larger plot sizes under the CL and AF treatment was better for the tractor to plow the soils before sown. Besides, the larger plot sizes were needed to sow suffice seeds for corn and alfalfa because they were sown with row spacing of 65 and 25 cm, respectively. More detailed descriptions of the land use treatments are shown in Yu et al. (2017 and 2018). 2.3. Soil sampling and analyses Soil samples were taken in early September 2015 at two soil depths (0 to10 cm and 10 to 20 cm) with a 4-cm diameter soil core sampler. Five individual soil samples selected at least 6 m apart from one another from each land use treatment were gently mixed to obtain a composite soil sample for each soil depth. In total, 40 composite samples were obtained, including five land use treatments, two soil depths and four replicates. The root materials and other visible debris were separated from the soil samples by hand. One sub-sample was stored field moist in a cooler at 4 °C for microbial biomass carbon (MBC) analyses, and another sub-sample was air-dried, passed through a 2-mm sieve for soil enzyme activities analyses. The activities of four soil enzymes were analyzed using the standard methods (Guan, 1986; Yu et al., 2017). Catalase (EC 1.11.1.6, CAT) activity was determined by back-titrating residual H2O2 with a standard solution of KMnO4. Briefly, 3 g soil sample with 40 ml of DI water and 5 ml 0.30% H2O2 were added in a triangular flask and shaken for 30 min. The residual H2O2 was stabilized using 5 ml of 1.5 M H2SO4 and 25 ml of this solution was titrated with 0.05 M KMnO4 until the solution changed to pink. Urease (EC 3.5.1.5, URE) activity was measured by incubating 2 g soil sample with 10 ml of 10% urea solution for 24 h at 37 °C and the concentration of NH3-N released was colorimetrically at 578 nm. For the determination of alkaline phosphatase (EC 3.1.3.1, ALP) activity, 2 g soil

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sample was mixed in 0.4 ml toluene, borate buffer (pH = 9.0) and 10 ml disodium phenyl phosphate solution as substrate and was incubated at 37 °C for 12 h. After the incubation, the concentration of phenol released over 12 h was assayed colorimetrically at 510 nm. Invertase (EC 3.2.1.26, INV) activity was determined by incubating 3 g soil sample with 15 ml of 8% sucrose, 5 ml of phosphate buffer (pH = 5.5) and six drops of toluene for 24 h at 37 °C. The concentration of glucose released was assayed using a spectrophotometer at 508 nm. All reported values were expressed on oven-dry soil mass basis per specified time as the absolute enzyme activity. The MBC was determined using the chloroform fumigation-extraction method (Jenkinson and Powlson, 1976). The soil specific enzymes activities were calculated by dividing the absolute enzymes activities over the MBC contents (Zhang et al., 2015a; Raiesi and Salek-Gilani, 2018). 2.4. Statistical analysis Statistical analysis was carried out using SPSS 16.0 (SPSS Inc., Chicago, IL, USA). Before statistical analysis, the normality and equal variance of all data was tested to meet the assumptions of statistical analysis. The influences of land use conversions on each individual soil specific enzyme activity, soil absolute enzyme activity and MBC were analyzed using One-way analysis of variance (ANOVA). Differences among the mean values were tested by the Fisher's least significant difference test (P b 0.05). 3. Results 3.1. Changes in soil microbial biomass carbon and soil absolute enzymes activities under different land use treatments Contents of MBC at the 0 to 10 cm depth were higher than that at the 10 to 20 cm depth (Table 1). Land use conversions had significant effect on MBC content at 0 to10 cm depth, however the effect was not significant at the 10 to 20 cm depth. The highest MBC content at 0 to10 cm depth was found under the AF treatment, which was significant higher than that under CL and AG M treatment. Similar with MBC content, the four soil absolute enzyme activities at the 0 to 10 cm depth were higher than that at the 10 to 20 cm depth (Table 1). Land use conversions significantly affected the absolute activities of INV at the 0 to10 and 10 to 20 cm depth. However, the effects of land use conversions on the absolute activities of ALP were not significant at the 0 to 10 and 10 to 20 cm depth. The absolute activity of INV

Table 1 Changes in soil microbial biomass carbon and soil absolute enzyme activities at 0 to 10 and 10 to 20 cm depth under different land uses. Results are shown as mean (± SE). Values with same lowercase letters within land uses are not significantly different at P b 0.05. CL, cropland; AF, alfalfa perennial forage; AG M, monoculture grassland for hay once a year; AG, monoculture grassland; RG, successional regrowth grassland. Land use treatments

Soil microbial biomass carbon content

Catalase

Urease

Invertase

Alkaline phosphatase

(mg kg−1)

(ml KMnO4 g−1 soil 24 h−1)

(mg NH3-N g−1 soil 24 h−1)

(mg glucose g−1 soil 24 h−1)

(mg phenol g−1 soil 24 h−1)

0 to 10 cm CL AF AG AG M RG F P

295.83(±10.13)b 533.29(±16.50)a 427.59(±58.54)ab 391.07(±73.23)b 395.19(±30.52)ab 3.60 0.046

40.34(±1.19)b 43.13(±0.92)ab 43.53(±1.19)a 44.72(±1.13)ab 45.52(±0.062)a 2.85 0.061

0.53(±0.04)c 1.17(±0.10)a 0.78(±0.08)bc 0.63(±0.09)bc 0.83(±0.14)b 6.45 0.003

8.17(±0.88)b 15.35(±0.11)a 15.84(±0.65)a 17.04(±0.76)a 16.81(±0.91)a 25.86 0.000

1.43(±0.10)b 2.42(±0.10)a 2.00(±0.29)ab 2.18(±0.24)a 2.17(±0.34)a 2.50 0.087

10 to 20 cm CL AF AG AG M RG F P

162.09(±33.15)ab 256.14(±19.26)a 148.39(±31.32)b 117.25(±13.57)b 213.30(± 51.20)ab 2.89 0.079

31.59(±2.71)b 39.15(±0.46)a 39.95(±3.28)a 42.33(±2.25)a 36.76(±1.13)ab 3.37 0.037

0.36(±0.04)b 0.52(±0.02)a 0.43(±0.05)ab 0.45(±0.03)ab 0.44(±0.05)ab 1.84 0.174

1.78(±0.49)c 6.01(±0.36)ab 4.93(±0.59)b 7.13(±0.76)a 6.23(±0.86)ab 10.52 0.000

0.92(±0.08)a 1.43(±0.18)a 1.00(±0.08)a 1.08(±0.16)a 1.33(±0.28)a 1.55 0.238

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under AF, AG, AG M and RG treatment increased by 88%, 94%, 109% and 106% for 0 to 10 cm depth and by 238%, 177% 301% and 250% for 10 to 20 cm depth, respectively, compared with CL treatment. The absolute activity of INV at 10 to 20 cm depth under AG M treatment was significantly higher than that under the AG treatment. The absolute activity of URE at 0 to 10 cm depth was sensitive to changes in land use treatment, while the absolute activity of CAT at 10 to 20 cm depth was sensitive to land use treatment (Table 1). The URE activity at 0 to 10 cm depth under the AF treatment was significantly higher than that under RG and CL treatment. At 10 to 20 cm depth, the CAT activity under CL treatment was significantly lower than that under AF, AG and AG M treatment. 3.2. Changes in soil specific enzyme activities under different land use treatments Compared with the enzyme activities at the 0 to 10 cm depth, the soil specific enzyme activities at the 10 to 20 cm depth were higher, irrespective to the land use treatments. A significant effect (P b 0.05) of land use conversions on soil specific activities were found for the activity of CAT and INV (Fig. 1). At the 0 to 10 cm depth, the specific activity of CAT under CL treatment was highest, and it was significantly higher than that under AF and AG M treatment. However, the specific activity of CAT under CL treatment was significantly lower than that under AG treatment and it has no significant differences with other land uses at 10 to 20 cm depth. Conversions of cropland to re-vegetation land increased the specific activity of INV. The specific activity of INV under CL treatment at 10 to 20 cm depth was significantly lower than that under AF, AG M, AG and RG treatment, however, only the activities of INV under AG and RG treatment at 0 to 10 cm depth were significantly higher than that under CL treatment. Under AF, AG M, AG and RG treatment, the specific activities of INV at 0 to 10 cm depth (4.3%, 36.7%, 62.8% and 56.6%, respectively) and 10 to 20 cm depth (122.5%, 226.5%, 475.0% and 187.1%, respectively) were higher than that under CL treatment. Land uses had significant effect on the specific activity of URE at 10 to 20 cm depth, however the effect was not significant at 0 to 10 cm depth (Fig. 1). The specific activity of URE at 10 to 20 cm depth under AG was significantly higher as compared to CL, AF and RG treatment. Compared with other soil enzymes, influences of land uses on the specific activities of ALP at 0 to 10 cm and 10 to 20 cm depth were less pronounced (Fig. 1). Although land use conversions had no significant effects on the specific activity of ALP at 10 to 20 cm depth, the specific activity of ALP under AG treatment was significantly higher than that under CL and AF treatment. 3.3. Changes in the geometric mean and the sum of specific enzyme activities under different land use treatments

Fig. 1. Soil specific enzyme activities at 0 to 10 cm and 10 to 20 cm depth under different land uses. Values with same lowercase letters within land uses are not significantly different at P b 0.05. NS represent no significant differences among the treatments. The bars represent standard errors. CL, cropland; AF, alfalfa perennial forage; AG M, monoculture grassland for hay once a year; AG, monoculture grassland; RG, successional regrowth grassland.

Our results showed that responses of soil specific enzymes activities to land use conversions varied with different land use treatments and soil depths in this study (Fig. 1). A single measure of soil specific enzyme activity is not sufficient to expose the responses of soil enzyme activities to environmental changes (Yu et al., 2017). To better show the responses of soil specific enzyme activities to land-use conversions, two general indices, the geometric mean of specific enzyme activities (GME) and simply sum of specific enzyme activities (SEA) were calculated in this study followed the methods described by Raiesi and Beheshti (2014) and Yu et al. (2017). The effects of different land uses on GME differed among the soil depths (Fig. 2). There was no significant difference in GME at 0 to 10 cm depth among the different land uses. However, land uses had significant influences on GME at 10 to 20 cm depth. The GME value at 10 to 20 cm depth under AG treatment was highest, followed by AG M, which were significantly higher than that under CL and AF treatment. Responses of SEA values to land use conversions were same as the changes of GME (Fig. 3). Land use conversions had significant effects

P. Yu et al. / Science of the Total Environment 687 (2019) 939–945

Fig. 2. Changes in the geometric mean of soil specific enzyme activities (GME) under different land uses. Values with same lowercase letters within land uses are not significantly different at P b 0.05. NS represent no significant differences among the treatments. The bars represent standard errors. See Fig. 1 for abbreviations.

on the SEA at 10 to 20 cm depth and no significant differences were found at 0 to 10 cm depth among the land use treatments. The higher value of soil specific enzyme activity was found in the order of AG N AG M ≈ RG N AF N CL treatment. 4. Discussion 4.1. Effects of land-use conversions on the soil specific enzymes activities Changes in land uses are known to affect the soil enzyme activities by directly changing soil properties, biological communities as well as the inputs and outputs of organic matter into the soils (Raiesi and Beheshti, 2014; Ma et al., 2015; Xie et al., 2017; Silva et al., 2019). The specific enzyme activities differed under different land uses and soil depths (Fig. 1). Responses in the specific activities of CAT and INV to land use conversions were more sensitive as compared to that of URE and ALP in 0 to 10 cm depth. Conversions of cropland to re-vegetation land significantly increased the specific activities of INV in 0 to 10 cm depth, indicating re-vegetation had a positive effect on the improvement of INV activities. This is consistent with the previous findings showing the native or a less-disturbed soil has higher enzyme activities than the cropland (Wang et al., 2012b; Raiesi and Salek-Gilani, 2018). The specific activities of INV, the enzyme related to the C cycle, improved remarkably under AG and RG treatment because the inputs of organic matter into the soils under these two land uses were higher than other land use treatments (Wang et al., 2016; Zhao et al., 2016). Our previous study in the same site showed that the inputs of aboveground and belowground biomass into soils at 0 to 20 cm depth under

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CL, AF, AG M, AG and RG treatment were 137, 628, 638, 837 and 745 g m−2, respectively (Yu et al., 2017). In addition, the increase in the inputs of organic matter into soils can promote the formation of soil aggregates, and thus increasing the protection of stable aggregates to soil enzymes (Fansler et al., 2005). Differ from the INV, the specific activity of CAT under CL treatment was higher than that under other land uses. The enzyme of INV enable the peroxide produced during metabolism to decompose, thus preventing its toxic effects on organisms, and its activity is closely relevant to contents of soil organic carbon and available nitrogen (Zhang et al., 2015b; Sun et al., 2016). Therefore, the significant higher content of total available nitrogen under CL treatment due to the fertilization might be the primary reason for the higher CAT activity (Yu et al., 2019). Besides, the lower MBC content under CL treatment (Table 1) compared with the re-vegetation land might be another important reason for the higher CAT activity. More obvious influences of land use conversion on soil specific enzyme activities were found at 10 to 20 cm depth than that at 0 to 10 cm depth (Fig. 1). The specific activities of the four enzymes in 10 to 20 cm depth were found in the order of AG N AG M N RG N CL N AF treatment, indicating re-vegetation also improved the enzyme activities in the subsoil in northeastern China. The obvious increase of the specific enzyme activities under the re-vegetation, especially the AG treatment were mainly result from the higher continuous inputs of vegetation biomass and the better soil conditions and aeration conditions in the revegetation land than the cropland (Liu et al., 2014; Xie et al., 2017). The continuous inputs of organic matter in the re-vegetation land not only provides sufficient food for soil microorganisms growth, but also increase the sorption of enzymes and their substrates to organic matter surfaces and soil aggregates (Fansler et al., 2005; Raiesi and Salek-Gilani, 2018). Furthermore, the newly growing vegetation in re-vegetation land can produce substrates and secretions, which can also increase the enzymes activities (Zhang et al., 2012). Similarly, Li et al. (2018) in northwest China and Rui et al. (2014) in sub-tropical China also observed that re-vegetation had a positive effect on the improvement of soil enzyme activities. Compared with other land uses, the specific enzyme activities under AF treatment was lower than that under CL treatment except the INV. The two-year planting of alfalfa might be the primary reason because short-term land use has limiting effects on the recovery of soil enzyme activity and other soil properties (Yu et al., 2017). In addition, the higher MBC content under AF treatment (Table 1) might be another important reason for the lower enzyme activities as the CAT in re-vegetation land. Different with other soil enzymes activities, land use conversions had no significant effects on the ALP activity at both soil depths (Fig. 1). Soil pH was the primary reason for the changes of soil phosphatase. The phosphatase is mainly alkaline phosphatase when soil pH N7, otherwise the phosphatase is mainly neutral and acid phosphatase (Guan, 1986). Our previous study showed that the soil pH under the five land use treatment narrowly ranged from 9.0 to 9.4 for 0 to 10 cm depth and from 9.4 to 10.0 for 10 to 20 cm depth (Yu et al., 2018). The remarkably high and narrowly range of soil pH resulted in the minimal change in ALP activities in this study. 4.2. Responses of the GME and SEA to land use conversions

Fig. 3. Changes in the sum of soil specific enzyme activities (SEA) under different land uses. Values with same lowercase letters within land uses are not significantly different at P b 0.05. The bars represent standard errors. See Fig. 1 for abbreviations.

As a common index, the geometric mean could combine data and information from various soil properties with different units (Yu et al., 2017; Raiesi and Salek-Gilani, 2018). Previous studies have reported that the GME calculated based on the enzyme activities is a sensitive and useful index of soil quality and it can shows the overall enzyme activities as well as soil microbiological activities (Wang et al., 2012a; Raiesi and Beheshti, 2014; Raiesi and Salek-Gilani, 2018). Therefore, the GME was calculated using the specific enzyme activities in this study. Furthermore, the SEA was also calculated as a useful index of soil quality to show the response of soil enzyme activities to different land uses. Similar change trends of GME and SEA under different land

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uses at 0 to 20 cm depth (Figs. 2 and 3) were found in this study. Under AF, AG M, AG and RG treatment, GME (0.2%, 32.8%, 65.7% and 24.3%, respectively) and SEA (2.6%, 38.0%, 82.8% and 29.6%, respectively) were higher than that under CL treatment. These results indicated that revegetation had a positive effect to improve the enzyme activities, and this two indices of GME and SEA could be representative of changes in soil enzyme activities and quantified influences of land use conversions on soil quality. Our results showed that the GME and SEA did not change with landuse conversions at 0 to 10 cm depth, while the GME and SEA was significant different among the studied land uses in 10 to 20 cm depth (Figs. 2 and 3). Although the differences of GME and SEA at 0 to 20 cm depth were similar among the five land uses, the F value of SEA obtained from the ANOVA analysis was higher than that of GME. These results indicated that the index of SEA was more sensitive and useful for differentiating the changes in soil enzyme activities among land uses than the GME. Moreover, the index of SEA is simple and does not require complicated mathematical calculations. Therefore, the index of SEA is recommended as a comprehensive index to present the changes in soil quality when various soil properties are used to analyze the soil quality changes. Among the studied land use treatments, the highest values of SEA and GME at the 0 to 20 cm depth were all found under AG treatment, indicating land use of AG might be the optimal land use to quickly recovery soil enzyme activities in northeastern China. 4.3. Comparison of soil specific and absolute enzymes activities to land use conversions Several differences in the changes of soil enzyme activities with different land uses and soil depths were found between the soil absolute and specific enzymes activities. Contrary to the higher specific enzyme activities at 10 to 20 cm depth, soil absolute enzyme activities in the 0 to 10 cm depth were higher than that in the 10 to 20 cm depth under all the five land uses (Table 1). Similarly. The MBC contents at the 0 to 10 cm depth were also higher than that in the 10 to 20 cm depth. The similar change trend of soil absolute enzyme activities and the MBC content in the soil profile confirmed the significant correlation between them reported by the previous studies (Raiesi and Beheshti, 2014; Liu et al., 2017), and also demonstrated that soil enzymes activities as a product of per unit soil microorganisms were lower in the 0 to 10 cm depth than that in the 10 to 20 cm depth. Conversions from cropland to re-vegetation land significantly increased the absolute enzyme activities at 0 to 10 cm depth (Table 1), however, the influences of land-use conversions on the improvement of soil specific enzyme activities were less pronounced except the INV activities under AG and RG treatment (Fig. 1). At the 10 to 20 cm depth, the soil absolute enzyme activities under AF treatment were higher than that under CL treatment, whereas the specific enzyme activities were opposite. Besides, the amplitude of variations in soil absolute and specific enzyme activities under AG, AG M and RG treatment were remarkable different compared with CL treatment (Fig. 1, Table 1). These different responses of soil absolute and specific enzyme activities to land use conversions were mainly due to the diversity in the MBC under different land uses and soil depths (Katsalirou et al., 2010; Zhang et al., 2015b). Re-vegetation increased the MBC content at 0 to 10 cm and 10 to 20 cm depth by 47.65% and 13.38%, respectively. Corresponding to this, the GME of the soil absolute enzyme activities under re-vegetation land at 0 to 10 cm and 10 to 20 cm depth increased by 51.20% and 64.34% (Yu et al., 2017), respectively, while the GME of the soil absolute enzyme activities under re-vegetation land at 0 to 10 cm and 10 to 20 cm depth increased by 4.77% and 57.50% (Fig. 2), respectively, compared with CL treatment. Besides, the changes in the MBC content under different land uses also resulted in that the highest value of soil absolute and specific activities for the same enzyme was found under different land uses. For example, the highest specific activities of CAT, URE, INV and ALP at 10 to 20 cm depth were all found under

AG treatment. However, the highest absolute activities of CAT and INV at 10 to 20 cm depth were found under AG M treatment, and the highest values of URE and ALP were found under AF treatment. Similar, Liu et al. (2017) in Henan province, China and Raiesi and Salek-Gilani (2018) in Chaharmahal and Bakhtiari province, Iran also reported that the responses of soil absolute and specific enzyme activities were different with different soil management practices. Results of our previous study (Yu et al., 2017) based on the soil absolute enzyme activities showed that no significant differences among the forage and grasslands were found in the 0 to 20 cm depth except the URE activity under AF treatment at 0 to 10 cm depth and the INV activity under AG M treatment at 10–20 cm depth (Table 1). However, differences of the specific enzyme activities among the forage and grasslands were notable, especially at the 10 to 20 cm depth (Fig. 1). Furthermore, the ANOVA results of the specific enzyme activities showed that F values and levels of significance were higher than that of the absolute enzyme activities except the URE, INV and ALP activity at 0 to 10 cm depth. These findings are in agreement with the results of Raiesi and Beheshti (2014) and Zhang et al. (2015a), who report the responses of soil specific enzyme activities to land use changes are more clearly than the absolute enzyme activities. The soil specific enzyme activity is a reflection of enzyme efficiency and its change is closely related to the shifts in enzyme production and decomposition of enzymes (Allison et al., 2007; Raiesi and Salek-Gilani, 2018). Consequently, the specific enzyme activity is recommended to use as a useful and sensitive indicator for ecological changes of soils after land use conversions in semiarid agroecosystems.

5. Conclusions Results in this study indicated that the specific enzyme activity of alkali soils were influenced by short-term land use conversions in northeastern China, however the direction and magnitude of changes in soil specific enzyme activities were different under different land uses. Differ from the soil absolute enzyme activities, influences of land use conversions on the specific enzyme activities at 10 to 20 cm depths were more obvious than that at 0 to 10 cm depth, and the higher specific enzyme activities were found at 10 to 20 cm depth, irrespective to the land use treatments. The higher values of GME and SEA for the specific enzyme activities under land uses of AG M, AG and RG as compared to CL treatment indicated that conversion of cropland to re-vegetation land increased the soil enzyme activities in northeastern China. The soil specific enzyme activity is a more sensitive and appropriate indicator reflecting the changes of alkali soils following land use conversions due to better discrimination and the higher F value and significance level of ANONA analysis than the absolute enzyme activities. According to the highest GME and SEA value under the AG treatment, land use of AG may be the best agricultural management practices to rapidly improve the soil enzymes activities in northeastern China. The present study draws the following conclusions: (1) Conversion of cropland to grassland increases the soil enzyme activities and the soil quality could be gradually recover following re-vegetation in northeastern China; (2) Response of soil specific enzyme activities was more clearly to short-term land use conversion than the absolute enzyme activities and it can be used as a sensitive indicator to monitor the change in soil quality after land use changes.

Funding This work was supported by the National Natural Science Foundation of China [31500446 and 41601124], the Fundamental Research Funds for the Central Universities (SWU019024 and SWU019023), Excellent Young Foundation of Jilin Province (20190103141JH) and University Innovation Research Group of Chongqing (Remote sensing of fragile ecological environment in southwest China).

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