Change of soil productivity in three different soils after long-term field fertilization treatments

Journal of Integrative Agriculture 2020, 19(3): 848–858 Available online at www.sciencedirect.com

ScienceDirect

RESEARCH ARTICLE

Change of soil productivity in three different soils after long-term field fertilization treatments LIU Kai-lou1, 2, HAN Tian-fu1, HUANG Jing1, ZHANG Shui-qing3, GAO Hong-jun4, ZHANG Lu1, Asad Shah1, HUANG Shao-min3, ZHU Ping4, GAO Su-duan5, MA Chang-bao6, XUE Yan-dong6, ZHANG Huimin1 1

National Engineering Laboratory for Improving Quality of Arable Land/Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, P.R.China 2 National Engineering and Technology Research Center for Red Soil Improvement/Jiangxi Institute of Red Soil, Nanchang 331717, P.R.China 3 Institute of Plant Nutrition and Environmental Resources Science, Henan Academy of Agricultural Sciences, Zhengzhou 450002, P.R.China 4 Institute of Agricultural Resource and Environment, Jilin Academy of Agricultural Sciences, Changchun 130033, P.R.China 5 San Joaquin Valley Agricultural Sciences Center, USDA Agricultural Research Service, CA 93648-9757, USA 6 Cultivated Land Quality Monitoring and Protection Center, Ministry of of Agriculture and Rural Affairs, Beijing 100125, P.R.China

Abstract Soil productivity (SP) without external fertilization influence is an important indicator for the capacity of a soil to support crop yield. However, there have been difficulties in estimating values of SPs for soils after various long-term field treatments because the treatment without external fertilization is used but is depleted in soil nutrients, leading to erroneous estimation. The objectives of this study were to estimate the change of SP across different cropping seasons using pot experiments, and to evaluate the steady SP value (which is defined by the basal contribution of soil itself to crop yield) after various longterm fertilization treatments in soils at different geographical locations. The pot experiments were conducted in Jinxian of Jiangxi Province with paddy soil, Zhengzhou of Henan Province with fluvo-aquic soil, and Gongzhuling of Jilin Province with black soils, China. Soils were collected after long-term field fertilization treatments of no fertilizer (control; CK-F), chemical fertilizer (NPK-F), and combined chemical fertilizer with manure (NPKM-F). The soils received either no fertilizer (F0) or chemical fertilizer (F1) for 3–6 cropping seasons in pots, which include CK-P (control; no fertilizer from long-term field experiments for pot experiments), NPK-P (chemical fertilizer from long-term field experiments for pot experiments), and NPKM-P (combined chemical and organic fertilizers from long-term field experiments for pot experiments). The yield data were used to calculate SP values. The initial SP values were high, but decreased rapidly until a relatively steady SP

Received 30 November, 2018 Accepted 20 May, 2019 LIU Kai-lou, E-mail: [email protected]; Correspondence ZHANG Hui-min, Tel: +86-10-82105039, E-mail: zhanghuimin@ caas.cn © 2020 CAAS. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). doi: 10.1016/S2095-3119(19)62742-5

LIU Kai-lou et al. Journal of Integrative Agriculture 2020, 19(3): 848–858

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was achieved at or after about three cropping seasons for paddy and fluvo-aquic soils. The steady SP values in the third cropping season from CK-P, NPK-P, and NPKM-P treatments were 37.7, 44.1, and 50.0% in the paddy soil, 34.2, 38.1, and 50.0% in the fluvo-aquic soil, with the highest value observed in the NPKM-P treatment for all soils. However, further research is required in the black soils to incorporate more than three cropping seasons. The partial least squares path mode (PLS-PM) showed that total N (nitrogen) and C/N ratio (the ratio of soil organic carbon and total N) had positive effects on the steady SP for all three soils. These findings confirm the significance of the incorporation of manure for attaining high soil productivity. Regulation of the soil C/N ratio was the other main factor for steady SP through fertilization management. Keywords: manure incorporation, C/N ratio, soil types, grain yield

1. Introduction There is a great need to evaluate soil productivity (SP) without the influence of fertilizers because it is highly associated with the resilience of a soil. Many long-term fertilization experiments have reported the possibility of some crop yield without fertilizer (Li et al. 2004; Zhang et al. 2009, 2010; Duan et al. 2011). Fan et al. (2013) indicated that the crop yield in control plots without fertilizer addition under on-farm trials in the 2000s was significantly higher compared with those achieved in the 1980s for major irrigated cereal-based cropping systems. However, the crop yield in control plots without fertilizer addition varied hugely across different soil types due to a diversity of crop varieties, climatic conditions, soil fertility levels, and other factors (Fan et al. 2013; Gong et al. 2013; Zha et al. 2014; Liao et al. 2016). In general, the crop yield in these control plots is controlled by SP, which represents the ability of soil to supply crop yield under a specific climate or in a geographical region. At present, there exists little research on the SP of different soil types, although many studies have shown that SP can change across different fertilization treatments in the same soil type (e.g., black, fluvo-aquic, and paddy soils) (Gong et al. 2013; Zha et al. 2014; Liao et al. 2016). Different from soil fertility, SP integrates a fertility index estimated with soil chemical and biological indexes such as soil pH, soil organic carbon (SOC), soil total or available nitrogen (N), phosphorus (P) and potassium (K), earthworm presence, and others (Liu et al. 2014; Liu et al. 2015; Bünemann et al. 2018). SP is quantified as the crop yield without fertilizer input to the soil (Gong et al. 2013; Zha et al. 2014). In general, the higher the SP, the higher the crop yield, and the fewer chemical fertilizers needed for a target yield (Zeng et al. 2012). Estimation of SP provides theoretical and technical support for chemical fertilizer reduction and synergism in China and elsewhere. Longterm fertilization experiments have been widely used to develop fertilization and management strategies to sustain high crop yield and improve SP (Li et al. 2004; Zhang et al.

2009, 2010; Duan et al. 2011). Fertilization is essential for crop yield, but the amount of fertilizer required depends on the SP levels because of variation in soil properties, rainfall, and temperature (Yang 2006; Cong et al. 2012). Thus, estimation of the SPs assists in development of long-term nutrient management strategies. Different approaches are used to estimate SP, which include the productivity index (PI), and the Decision Support System for Agrotechnology Transfer (DSSAT ) model as well as using direct yield data from field or pot experiments. The PI method requires input of a large amount of data including soil physical and chemical properties for weighted analysis, and some parameters need to be modified for different soil types (Duan et al. 2009; Sun et al. 2009). The DSSAT model requires input of meteorological data and genetic properties of selected crop variety and it is often difficult to validate the simulation result (Zha et al. 2014). Pot experiment in paddy soil is a straightforward approach using direct yield data to estimate SP (Lu et al. 2015). However, it is difficult to obtain a steady SP especially for long-term fertilization soils without external fertilization using only a one-year (two cropping seasons) pot experiment, and overestimates of the SP were observed in paddy soils (Lu et al. 2015; Liao et al. 2016) because of the residual fertilizer influence (Wang et al. 2012). The steady SP, without fertilizer influence, is defined by the basal contribution of soil itself to crop yield. We hypothesized that this problem should be resolved by increasing the number of cropping seasons in pot experiments until a relatively steady SP was obtained. Evaluation of the steady SP is especially valuable from long-term fertilization treatments that are used to develop effective fertilization or management strategies. However, the steady SP value will be overestimated due to residual nutrient from chemical fertilizers of previous crops. In general, the crop yield is expected to be higher in early cropping seasons because of the readily available or residual fertilizer remaining in soil, but it would be reduced rapidly to a relatively stable level after the residual fertilizers are depleted, and further depletion of soil nutrients would result in crop yield declines. The SP will change accordingly

CK-F, control, no fertilizer in long-term field experiments; NPK-F, chemical fertilizer in long-term field experiments; NPKM-F, combined chemical and organic fertilizers in long-term field experiments. 1)

82.5 82.5 82.5 82.5 165 50 130 93.8 93.8 93.8 93.8 187.5 57.5 115 82.5 82.5 82.5 82.5 165 50 40.5 75 75 45 45 90 90 27 75 75 45 45

N N

90 90 CK-F NPK-F NPKM-F

Treatment

This research was conducted at three sites from south to north in China, located at Jinxian (116°10´E, 28°21´N) in Jiangxi Province, Zhengzhou (113°66´E, 34°76´N) in Henan Province, and Gongzhuling (124°82´E, 43°50´N) in Jilin Province, China. Long-term experiments were established at Jinxian, Zhengzhou, and Gongzhuling in 1981, 1990, and 1990, respectively. More detailed information for the long-term experiments can be found in previous reports (Zhang et al. 2009; Deng et al. 2014; Liu et al. 2019b). The soils are paddy (Stagnic Anthrosols), fluvo-aquic (Alluvic Entisols) and black (Vertisols) soils, at the three study sites, respectively, based on IUSS classification (IUSS Working Group 2006). These soils are important types for grain production including rice, wheat, and maize in China (Pan et al. 2009; Zhang et al. 2013; Chen et al. 2014). For the purpose of this study, SP was evaluated for three long-term field fertilization treatments: no fertilizer (CK-F), chemical fertilizer (NPK-F), and combined chemical and organic fertilizers (NPKM-F) for all three sites by conducting the pot experiments adjacent to these long-term field experiments. The fertilizer amounts applied to the CK-F, NPK-F and NPKM-F treatments in the long-term field experiments are shown in Table 1. The cropping system was early and late rice (Oryza sativa) in Jinxian site, wheat (Triticum aestivum)-maize (Zea mays) rotation in Zhengzhou site, and single maize in Gongzhuling site. Information on the soil properties and cropping systems of long-term fertilization experiments at each site is also provided in Table 1. Crops grown in the pot experiments were the same as in the field experiments. There were double cropping systems for the paddy soil at Jinxian and fluvo-aqui soil at Zhengzhou, and a single cropping system for black soil at Gongzhuling. The climates are classified as sub-tropical, humid in Jinxian, warm-temperate, semi-humid in Zhengzhou, and mild-temperate, semi-humid in Gongzhuling, with mean temperatures of 18.5, 14.5, and 5.5°C; annual cumulative temperature (>10°C) of 3 500, 2 661, and 1 700°C; mean annual precipitation of 1 450, 615, and 589 mm; mean annual evaporation of 1 650, 1 450, and 1 400, respectively. The pot experiments were conducted from 2012 to 2014 for all three locations. The early and late rice varieties in Jinxian were You I156 and Xiangfengyou 9, the winter wheat and summer maize varieties in Zhengzhou were Zhengmai 0856 and Xundan 20, and the spring maize variety in Gongzhuling was Xianyu 335. There were a total of 6, 5, and 3 seasons in the paddy, fluvo-aquic, and black soils, respectively. The cropping seasons differed among the three soils because of the different growing seasons for the crops at the various locations. Rice was planted by transplanting 3 plants in each pot, wheat and maize with sowing 3 seeds in each pot. Early rice was grown from April to July, and late rice was from August to November at Jinxian; winter wheat was grown from November to June, and summer maize was from June to October at Zhengzhou;

Table 1 Fertilizer application per planting season (kg ha–1) in three long-term field experiments in China

2.1. Study sites and soils

Paddy soil (early and late rice, Jinxian, Jiangxi Province) Early rice Late rice N P2O5 K2O Manure-N P2O5 K2O Manure-N

2. Materials and methods

Fluvo-aquic soil (wheat and maize, Zhengzhou, Henan Province) Winter wheat Summer maize P2O5 K2O Manure-N N P2O5 K2O Manure-N

Black soil (maize, Gongzhuling, Jilin Province) Single maize N P2O5 K2O Manure-N

with the yield, i.e., after the initial stage of fast decrease as illustrated in Lu et al. (2015) and Liao et al. (2016) for the early and late rice cropping system in paddy soils, a relatively steady SP should be achieved, representing the ability of the soil to support crop yield. Liu et al. (2019a) also proved that SP of paddy soil would changed among different fertilization years in one site, but the change of SP in different seasons was not analyzed, especially comparing with other sites. The objectives of this study were to estimate the trend of SP values across different cropping seasons using pot experiments, and to compare the difference in steady SP values after various long-term fertilization treatments in soils at different cropping systems or geographical locations. Therefore, this research will provide assistance in fertilization management at different soil fertility levels in the three soil types.

115

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1)

850

851

1.25 1.25 1.25 2.5 2.5 2.5 0 0 0 0 0 0 0 0 0 0.48 0.48 0.48 0.48 0.48 0.48 1.2 1.2 1.2 1.12 1.12 1.12 1.12 1.12 1.12 2.8 2.8 2.8 0 0 0 0 0 0 0 0 0

F0, no fertilizer in pot experiments; F1, chemical NPK fertilizers in pot experiments. In paddy soil, the crop of seasons 1, 3, and 5 was early rice; the crop of 2, 4, and 6 was late rice. In fluvo-aquic soil, the crop of seasons 1, 3, and 5 was summer maize, the crop of 2 and 4 was winter wheat. In black soil, the maize of seasons 1, 2, and 3 was maize. 2) CK-P, no fertilizer from long-term field experiments for pot experiments (control); NPK-P, chemical fertilizer from long-term field experiments for pot experiments; NPKM-P, combined chemical and organic fertilizers from long-term field experiments for pot experiments.

To compare the soil properties across different fertilization treatments, field soil sampling and analysis was conducted. Soil samples were collected from the plough horizon (0–20 cm) before each cropping season and after the pot experiments. In each pot, five cores were randomly taken and mixed into one sample. The soil samples were air-dried and passed through a 0.15-mm sieve to determine SOC using a wet oxidation method with potassium dichromate and concentrated sulfuric acid; total N, total P, and total K contents were measured by the micro-Kjeldahl, molybdenum, and flame photometric methods (Lu 2000). Another subsample was passed through a

1)

2.4. Evaluation of soil fertility factors

3.04 3.04 3.04

where SP is soil productivity in different treatments (%). YieldF0 and YieldF1 are the grain yield (g/pot) in F0 and F1 conditions, respectively. T is the treatment for pot experiment from the long-term field experiment, i.e., CK-P, NPK-P, and NPKM-P treatments. The SP of pot experiments was estimated for each cropping season for a total of 6, 5, and 3 seasons (during three years) at Jinxian, Zhengzhou, and Gongzhuling, respectively.

2.03 2.03 2.03

(1)

3.38 3.38 3.38

×100 

0 0 0

YieldF1(T)

0 0 0

YieldF0(T)

0 0 0

SP (%)=

CK-P NPK-P NPKM-P

For the pot experiments, the SP was calculated based on the definition:

Treatment2)

2.3. Estimation of SP

Paddy soil (early and later rice, Jinxian, Jiangxi Province ) Season 1–6 in F0 Season 1–6 in F1 K2O N P2O5 K2O N P2O5

Surface soils (0–20 cm depth) were collected from the three long-term field fertilization treatments (CK-F, NPK-F, and NPKM-F) to conduct pot experiments at each study site. These soils for pot experiments were sampled in late November, 2011 (after harvesting late rice) at Jinxian site, early October, 2011 (after harvesting summer maize) at Zhengzhou site, and late September, 2011 (after harvesting single maize) at Gongzhuling site. The soils were air-dried, sieved to pass through 2 mm, and mixed before placing into pots. For each soil sample for the field fertilization treatment, six pots were prepared. Each pot (25 cm diameter and 35 cm height) was filled with 15 kg air-dry soil to 10 cm below the top edge. The pot experiments in three sites were included CK-P (control; no fertilizer from long-term field experiments for pot experiments), NPK-P (chemical fertilizer from long-term field experiments for pot experiments), and NPKM-P (combined chemical and organic fertilizers from long-term field experiments for pot experiments). For the CK-P, NPK-P, and NPM-P treatments, three pots received no fertilizer (F0), and the other three received chemical NPK fertilizers (F1) (Table 2). In addition, the amounts of chemical NPK fertilizers differed between winter wheat and summer maize due to different nutrient uptake. Soil in the F0 and F1 pots was mixed in the same long-term fertilization treatment after each cropping season. Visible pieces of crop residues and roots were removed, and 100 g soil of the long-term fertilization treatment was sampling for analyzing soil properties. Mixed soil samples were divided into six pots, and the experimental design of the six pots was same as in the first cropping season (Table 1). Planting and harvesting in the pot experiments followed the same schedule as the field experiments. At harvest, plant samples were collected, and panicles were hand-threshed and oven-dried at 80°C until constant weight to determine grain yield.

Table 2 Fertilizer application (g/pot) in pot experiments to estimate soil productivity1)

2.2. Pot experiments and measurements

Fluvo-aquic soil (wheat and maize, Zhengzhou, Hennan Province) Season 1–5 in F0 Season 1, 3 and 5 in F1 Season 2 and 4 in F1 N P2O5 K2O N P2O5 K2O N P2O5 K2O

Black soil (maize, Gongzhuling, Jilin Province) Season 1–3 in F0 Season 1–3 in F1 N P2O5 K2 O N P2O5 K2O

maize was grown from April to September at Gongzhuling.

1.25 1.25 1.25

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LIU Kai-lou et al. Journal of Integrative Agriculture 2020, 19(3): 848–858

the contribution of soil fertility factors (before the third cropping season) to steady SP in the third cropping season was analyzed using the Random Forest model (R software package vegan). The complex interrelationships between soil types, fertilization, soil fertility factors, and steady SP were shown by a partial least squares path mode (PLS-PM) using the R Software package vegan.

1-mm sieve for determination of available N, Olsen P, and available K (Lu 1999).

2.5. Statistical analyses One way analysis of variance (ANOVA) was performed to test the effects of fertilization treatments on grain yield and soil physical and chemical properties in each pot experiment by SPSS Software. The least significant difference (LSD at P<0.05) test was applied to assess the differences. In this study, the steady SP values were found in the third cropping season for paddy and fluvo-aquic soils. In addition, soil fertility before the third cropping season was more important in predicting the steady SP than after the third cropping season. So, the soil samples in pot experiments were analyzed from each field treatment in the three soils before the third cropping season. Furthermore,

3. Results 3.1. Grain yield in pot experiment Grain yield in the pot experiments varied across different cropping seasons due to crop variety and interannual climate variations (Fig. 1). There were significant differences among the CK-P, NPK-P, and NPKM-P treatments, with a similar trend of NPKM-P>NPK-P>CK-P for all three soil types

CK-P

120

70 50 a

30

Grain yield (g/pot)

b c

10 0

90 80

30

a b b

c

a b b

a a a a a

a a b b

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c

20 0

140 a

b c

c

10 1 3 5 Early rice

2 4 6 Later rice

80

a a a b

b c

a b c

1 3 5 Summer maize

60 a a a a a b 2 4 6 Winter wheat

F1, fluvo-aquic soil (wheat and maize, Zhengzhou)

120

a

60

b

40

c

0

40

b

20 0

200 180 160

c 1

a

a

a

b c

b c

2 3 Maize

F1, black soil (maize, Gongzhuling) a

140

80

20

a

100

100 b

20 0

40

2 4 6 Later rice

a

120

60 a b

F1, paddy soil (early and late rice, Jinxian)

60 40

c

b

1 3 5 Early rice

70 50

a b

140

80

a

F0, black soil 200 (maize, Gongzhuling) 180 160

100

60

20

NPKM-P

F0, fluvo-aquic soil 140 (wheat and maize, Zhengzhou)

F0, paddy soil 90 (early and late rice, Jinxian) 80

40

NPK-P

b

a

120

b

100

a

80

b

a b

c

c

c

a a

1 3 5 Summer maize

a a a

a a a

2 4 6 Winter wheat

60 40

c

20 0

1

2 3 Maize

Fig. 1 Grain yield in pot experiments to estimate soil productivity after different field fertilization treatments in Jinxian (Jiangxi Province), Zhengzhou (Henan Province) and Gongzhuling (Jilin Province), China. F0, no fertilizer in pot experiments; F1, chemical NPK fertilizers in pot experiments; CK-P, no fertilizer from long-term field experiments for pot experiments (control); NPK-P, chemical fertilizer from long-term field experiments for pot experiments; NPKM-P, combined chemical and organic fertilizers from long-term field experiments for pot experiments. Different letters in a cropping season indicate significance at the P<0.05 level among different treatments at the same study site. Bars are SD (n=3).

853

LIU Kai-lou et al. Journal of Integrative Agriculture 2020, 19(3): 848–858

3.2. SP estimation from pot experiments

(P<0.05), except for some cropping seasons in paddy and fluvo-aquic soils. Not only was the grain yield of NPK-P and

SP was estimated from pot experiments after long-term field fertilization (Fig. 2). For all three soils, if the initial SP was high, then it decreased with increasing cropping season number. The initial SP values in the first cropping season in the CK-P, NPK-P, and NPKM-P treatments were 37.8, 57.5, and 88.9% for paddy soil, 56.1, 86.6, and 68.0% for fluvo-aquatic soil, and 26.9, 24.6, and 61.8% for black soil, respectively. Initial SP in the first cropping season from the NPKM-P treatment was the highest among the three fertilization treatments for paddy and black soils, but for fluvo-aquic soil, the NPK-P treatment was higher than the NPKM-P treatment. For the paddy and fluvo-aquic soils, the SP decreased markedly from the first to the third season, and became relatively stable or little changed from the third to the fifth or sixth season. For the black soil, however, the SP from the NPKM-P treatment decreased significantly from the first to the third season, while little changes were seen for the CK-P and NPK-P treatments. The SP from the NPKM-P treatment for the black soil likely approached a stable level by the end of the three cropping seasons. Based on the SP changes observed with increasing cropping seasons,

NPKM-P treatments higher than in CK-P treatment due to the highly depleted soil nutrients in the CK-P treatment, but yields differed between NPKM-P and NPK-P treatments. For the paddy soil (annual rice-rice rotation), the rice yield from the first to the sixth seasons in NPKM-P treatment was higher than NPK-P treatment by 12.4–100.1% under the F0 condition and 7.1–48.3% under the F1 condition. For the fluvo-aquic soil (maize and wheat rotation), compared to the NPK-P treatment, the grain yield from first to the fifth seasons in NPKM-P treatment increased by 21.5–140.5% from F0 and 8.1–82.1% from F1 treated conditions. For the black soil (maize only), the grain yield from the first to the third seasons in NPKM-P treatment was 84.0–216.6% higher than NPK-P treatment from the F0 condition and 10.9–27.3% under the F1 condition. In addition, the yield increase in NPK-P and NPKM-P treatments from F0 was more than F1, when compared to the CK-P treatment. It indicated that soil fertility after long-term application of chemical fertilizers and manure was improved in all three soils, thus, these soil amendments could support higher yield under no-fertilization conditions. CK 100 90

Paddy soil (early and late rice, Jinxian) a

Fluvo-aquic soil 100 (wheat and maize, Zhengzhou)

90

a

80

Soil productivity (%)

100

80

70

c a b

50 40

a a a

50 a a b

c

c

a a b

30

20

20 2

3

4

5

6

a

50 a

c

a

60

b

40

30

1

a

60

c

70

b

b b

Black soil (maize, Gongzhuling)

90

a

80

70 60

NPKM

NPK

b a b a a

a a b

40

b

30

b

b

b

c

20 1

2 3 4 5 6 Cropping season

1

2

b b 3

4

Fig. 2 The changes in soil productivity with cropping seasons from pot experiments in Jinxian (Jiangxi Province), Zhengzhou (Henan Province) and Gongzhuling (Jilin Province), China. CK-P, no fertilizer from long-term field experiments for pot experiments (control); NPK-P, chemical fertilizer from long-term field experiments for pot experiments; NPKM-P, combined chemical and organic fertilizers from long-term field experiments for pot experiments. In paddy soil, the crop of seasons 1, 3, and 5 was early rice, the crop of seasons 2, 4, and 6 was late rice. In fluvo-aquic soil, the crop of seasons 1, 3, and 5 was summer maize, the crop of seasons 2 and 4 was winter wheat. In black soil, the maize of seasons 1, 2, and 3 was maize. Different letters in a cropping season indicate significance at the P<0.05 level among different treatments at the same study site. Bars are SD (n=3).

9.40±0.32 a 8.95±0.16 a 8.67±0.07 b 11.61±0.10 b 11.34±0.16 a 10.11±0.20 b 11.78±0.28 a 11.70±0.12 a 10.34±0.21 b

Available K (mg kg–1) 57.86±6.78 b 69.17±2.45 a 70.26±3.20 a 54.36±4.98 c 80.12±3.85 b 127.7±5.65 a 104.4±6.87 c 127.7±5.13 b 301.7±6.94 a Olsen P (mg kg–1) 6.26±2.77 c 12.37±4.65 b 65.71±4.38 a 1.17±0.57 c 24.33±1.33 b 51.43±1.67 a 2.87±3.41 c 29.75±0.87 b 79.43±4.54 a Available N (mg kg–1) 113.5±9.24 c 125.6±6.22 b 143.5±9.23 a 64.45±5.66 c 81.25±5.66 b 104.7±12.76 a 60.51±4.65 c 78.32±3.24 b 84.19±2.76 a Total K (g kg–1) 12.15±0.23 a 12.33±0.68 a 12.30±0.99 a 16.46±0.68 a 16.37±0.91 a 16.87±0.74 a 20.33±0.48 a 21.02±0.46 a 21.63±0.73 a Total P (g kg–1) 0.51±0.13 b 0.60±0.08 b 1.23±0.14 a 0.56±0.09 b 0.87±0.06 b 1.04±0.08 a 0.64±0.17 b 0.53±0.14 b 1.47±0.31 a Total N (g kg–1) 1.87±0.09 b 2.07±0.07 b 2.48±0.10 a 0.57±0.12 c 0.74±0.09 b 1.02±0.23 a 1.14±0.12 b 1.27±0.18 b 2.48±0.31 a 5.05±0.11 b 5.11±0.04 b 5.19±0.07 a 8.03±0.56 a 7.79±0.17 a 7.43±0.31 a 7.56±0.13 a 6.00±0.07 c 7.12±0.04 b CK-P NPK-P NPKM-P CK-P NPK-P NPKM-P CK-P NPK-P NPKM-P

CK-P, no fertilizer from long-term field experiments for pot experiments (control); NPK-P, chemical fertilizer from long-term field experiments for pot experiments; NPKM-P, combined chemical and organic fertilizers from long-term field experiments for pot experiments. Values are mean±SD (n=3). Different letters in a column indicate significance at P<0.05 level among different treatments at the same study site.

1)

Two stages of SP included initial and steady SP. In this study, the initial SP dramatically decreased from the first to third cropping seasons among paddy, fluvo-aquatic, and black soils. The results suggested that at least three cropping seasons are required for paddy and fluvo-aquatic soils to achieve a relatively steady SP value (Fig. 2). However, because of the single cropping system used in black soil, a steady SP was not observed

Black soil (maize, Gongzhuling, Jilin Province)

4.1. Estimation of SP in pot experiments

Fluvo-aquic soil (wheat and maize, Zhengzhou, Henan Province)

4. Discussion

Paddy soil (early and later rice, Jinxian, Jiangxi Province)

The random forest model analysis used in this study suggested that the key factors contributing to the steady SP were total N, available N, and SOC for paddy, fluvo-aquic soil and black soils, respectively (Fig. 3). Furthermore, among all soil fertility factors, the C/N ratio was more important to the steady SP than other soil fertility factors for all three soils (Fig. 3). The PLS-PM analysis (Fig. 4) showed that the total N (path coefficient=0.639) and C/N ratio (path coefficient=–0.798) had positive effects on the steady SP. These positive effects were mediated by soil types and fertilization which then affected the pH, SOC, total N, available N, and C/N ratio. In addition, fertilization had positive effects on the pH (path coefficient=–0.627) and available N (path coefficient=–0.472), which had negatively affected the steady SP.

SOC (g kg–1) 17.55±0.27 c 18.54±0.52 b 21.51±0.46 a 6.62±0.37 c 8.39±0.22 b 10.62±0.34 a 13.43±0.51 c 14.87±0.42 b 25.64±0.65 a

3.4. Contribution of soil fertility factors to steady SP

pH

In accordance with the observed steady SP values, soil fertility of the different fertilization treatments in the pot experiments was analyzed before the third cropping season (Table 3). SOC, total N, total P, available N, and Olsen P contents in the NPKM-P treatment were significantly higher than the other treatments, but not total K and available K contents in paddy soil, and pH and total K in fluvo-aquic and black soils. Compared with the NPK-P treatment, the SOC, total N, total P, available N, and Olsen P contents of NPKM-P treatment was higher than NPK treatment by 16.0, 19.8, 105.0, 14.3, and 431.2% in paddy soil, by 26.6, 37.8, 19.5, 28.8, and 111.4% in fluvo-aquic soil, and 72.4, 95.3, 177.4, 7.5, and 167.0% in black soil, respectively. Furthermore, compared with the NPK-P treatment, the SOC/total N (C/N) ratios in the NPKM treatment decreased significantly among all three soils.

Treatment1)

3.3. Soil fertility in different fertilization treatments of three soils

Soil (crop, site)

we hypothesize that the turning point of SP decline or the relatively steady SP was achieved by the third cropping season for paddy and fluvo-aquic soils, although the black soil may require data from more than three cropping seasons to confirm. Therefore, the steady SP in the third cropping season in the CK-P, NPK-P, and NPKM-P treatments were 37.7, 44.3, and 49.9% for paddy soil, 34.2, 38.1, and 41.2% for fluvo-aquatic soil, and 22.4, 24.3, and 43.5% for black soil, respectively. The steady SP in the third cropping season observed in the NPKM-P treatment was increased than that in the NPK treatment by 12.6, 8.3, and 79.3% for paddy, fluvo-aquic, and black soils, respectively.

C/N ratio

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Table 3 Soil fertility parameters before the third cropping season of the pot experiments for three soils from different long-term field treatments in China

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Fluvo-aquic soil (wheat and maize, Zhengzhou)

Soil fertility factors

Paddy soil (early and late rice Jinxian)

Black soil (maize, Gongzhuling)

All soils

Available K

Olsen P

Availabe N

pH

Total K

Available K

Total K

Olsen P

Olsen P

Total N

pH

Available K

pH

pH

Available K

Total K

Total P

Total P

Olsen P

SOC

C/N ratio

Total K

Total P

Total P

SOC

SOC

Total N

Total N

Availabe N

C/N ratio

C/N ratio

Availabe N

Total N

Availabe N

SOC

C/N ratio

0 2 4 6 8 10

0 2 4 6 8 10

0 2 4 6 8 10

0 5 1015 20

Increased in mean squared error (%)

Fig. 3 Measure of importance of each soil fertility factor to the steady soil productivity based on the random forest model. K, potassium; P, phosphorus; SOC, soil organic carbon; N, nitrogen.

Fertilization

Soil types

0.688 pH

0.627

0.472 SOC

Total N

Available N

C/N ratio

0.639 Steady SP

–0.798

Fig. 4 The complex interrelationships between soil types, fertilization, soil fertility factors, and steady soil productivity. SOC, soil organic carbon; N, nitrogen; SP, soil productivity. Dark lines indicate no significant effect; solid and dotted lines indicate positive and negative effects, respectively. The attached values of different indexes are mean path coefficients.

during the first three growing seasons. With the inclusion of more cropping seasons, it is expected that a relatively steady SP would be achieved in black soil (the same as in paddy and fluvo-aquatic soils). When residual fertilizer from long-term fertilization treatments was consumed in the pot experiment, a relatively steady SP would be achieved. Once the steady SP is exhausted, crop yield would decrease (Wang et al. 2012). The results also suggested that the steady SP improved by long-term manure treatment could potentially support three cropping seasons; yields were significantly higher from NPKM-P treatments than those from the NPK-P treatments for all soils. Furthermore, pot experiments including three soil types and more than

two cropping seasons have been shown to be a reliable method for estimating steady SP, especially after long-term fertilization treatments. Grain yield varies with cropping seasons due to unfavorable factors, such as drought, floods, pests, and diseases. Changes in grain yield loss would therefore also affect the steady SP values to some extent. In future studies, the denominator of the SP equation could be changed to “the average yield of the F1 condition in all treatments”, and comparisons of SP would be reasonable for different treatments. However, this requires further validation.

4.2. Steady SP values in different soils and comparisons among long-term fertilization treatments There were different steady SP values in different soil types, based on the results from the pot experiment. For the NPK-P treatment in the third cropping season, the steady SP values of paddy and fluvo-aquic soil were 44.3 and 38.1%, they were higher than that of black soil (24.3%). The results indicate that the steady SP declined the most in the black soil from intensive chemical fertilization. In the NPKM-P treatment, the steady SP was improved in all three soils, with the largest improvement observed in the black soil (Fig. 2). The steady SP from the NPKM-P treatment for the third cropping season was higher than that in the NPK-P treatment of the black soil by 79.3%, compared to 12.6 and 8.3% in paddy and fluvo-aquic soils. The results indicate that soil fertility in the black soil has decreased more

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dramatically than in other soils from chemical fertilization and that it is crucial to maintain steady SP by replenishing SOC. The results imply that long-term manure incorporation have improved steady SP for all three soils. Other studies also supported these findings, showing that the NPKM treatment improves steady SP (Zha et al. 2014; Lu et al. 2015). Li et al. (2016) reported that steady SP differed among the different regions in China, and the paddy soils in Yangtze River region had higher steady SP than did other regions. The improvement of steady SP with the NPKM treatment was also reported based on the DASSAT model (Gong et al. 2013; Zha et al. 2014). Based on SP calculation using direct yield data, our study has achieved similar conclusions: manure incorporation is one of the most efficient long-term management strategies to improve steady SP for all soils. However, further information is required for high fertility soils (e.g., black soil) where organic fertilizer is not used in nutrient management. In addition, it is unrealistic to drastically reduce or completely eliminate chemical fertilizer application in China due to the lower steady SP (24.3–44.3% for NPK-P treatment), which is lower than those observed in the USA and Europe (more than 70%) (Tang and Huang 2009; Sun et al. 2017). Therefore, manure application could be conducted continuously with chemical fertilizer to ensure higher steady SP for meeting future crop production needs.

4.3. Contribution of soil fertility factors to the steady SP The random forest model analysis used in this study suggested that the key factors determining steady SP were total N, available N, and SOC for paddy, fluvo-aquic, and black soils, respectively. However, the key soil fertility factors were slightly different from those found with the DASSAT model (Gong et al. 2013; Zha et al. 2014). The SOC and total N contents were found to significantly contribute to steady SP in winter wheat of fluvo-aquic soils (Gong et al. 2013); SOC was the key factor that affected steady SP in single maize of the black soil (Zha et al. 2014). One reason for this discrepancy may be the methods: steady SP in this study was measured with pot experiments with 3–6 cropping seasons, and the three fertilization treatments were less than the number in the DASSAT model (Gong et al. 2013; Zha et al. 2014). Across all included soil fertility factors, the PLS-PM analysis showed that the total N and C/N ratio had positive effects on steady SP. This indicates that the relationship between organic carbon (C) mineralization and chemical N fertilizer could play a role in steady SP improvement, although fertilization or agricultural management that

enriches soil with organic C and N input, such as manure incorporation, has improved the steady SP. In previous studies, the input of C substrates from manure was found to promote soil microbial community structure and contribute to C stabilization (Zhu et al. 2017), and increased N mineralization rate of organic C supplied available N for crop uptake (Islam et al. 1998). Therefore, it is necessary for attaining higher steady SP to apply chemical and organic fertilizers in soil with an appropriate C/N ratio. However, if not managed properly, high organic fertilizer input and excess N fertilizers could also lead increasing C emission and N loss in soils (Shang et al. 2011; Sharpley and Wang 2014; Skinner et al. 2014). This research emphasizes the necessity of organic fertilizer to improve the steady SP. How manure can be most effectively used to minimize environmental impact in different soils still requires more systematic and in-depth research.

5. Conclusion This study has illustrated that the steady SP can be achieved after three cropping seasons in pot experiments for paddy and fluvo-aquic soils. Organic fertilizer becomes essential to maintain higher steady SP in paddy, fluvo-aquic, and black soils. The key soil fertility factors of steady SP were total N, available N, and SOC for paddy, fluvo-aquic, and black soils, respectively. However, across all three soils, soil total N and C/N ratio contribute significantly to the improvement of steady SP. The regulation of C mineralization is thus important for improvement of steady SP with the application of chemical N fertilizer.

Acknowledgements This research was supported by the National Key Research and Development Program of China (2016YFD0300901 and 2016YFD0200101), the Fundamental Research Funds for Central Non-profit Scientific Institution of China (161032019035 and 161032019020), and the National Basic Research Program of China (973 Program) (2011CB100501). The authors thank our colleagues in Jiangxi, Henan, Jilin provinces for their technical support in conducting the three long-term field experiments.

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Executive Editor-in-Chief ZHANG Wei-li Managing editor SUN Lu-juan