Stratification and Storage of Soil Organic Carbon

RESEARCH ARTICLE

Stratification and Storage of Soil Organic Carbon and Nitrogen as Affected by Tillage Practices in the North China Plain Xin Zhao1☯, Jian-Fu Xue1☯, Xiang-Qian Zhang1, Fan-Lei Kong1,2, Fu Chen1, Rattan Lal3, Hai-Lin Zhang1* 1 College of Agronomy & Biotechnology, China Agricultural University; Key Laboratory of Farming System, Ministry of Agriculture of China, Beijing, China, 2 College of Agronomy, Sichuan Agricultural University, Chengdu, China, 3 Carbon Management & Sequestration Center, School of Environment and Natural Resources, The Ohio State University, Columbus, Ohio, United States of America

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☯ These authors contributed equally to this work. * [email protected]

Abstract OPEN ACCESS Citation: Zhao X, Xue J-F, Zhang X-Q, Kong F-L, Chen F, Lal R, et al. (2015) Stratification and Storage of Soil Organic Carbon and Nitrogen as Affected by Tillage Practices in the North China Plain. PLoS ONE 10(6): e0128873. doi:10.1371/journal.pone.0128873 Academic Editor: Wenju Liang, Chinese Academy of Sciences, CHINA Received: March 12, 2015 Accepted: May 3, 2015 Published: June 15, 2015 Copyright: © 2015 Zhao et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper. Funding: This work was funded by Chinese Universities Scientific Fund (2014FG055), Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-13-0567) and Special Fund for Agro-scientific Research in the Public Interest in China (201103001). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Tillage practices can redistribute the soil profiles, and thus affects soil organic carbon (SOC), and its storage. The stratification ratio (SR) can be an indicator of soil quality. This study was conducted to determine tillage effects on the profile distribution of certain soil properties in winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.) systems in the North China Plain (NCP). Three tillage treatments, including no till (NT), rotary tillage (RT), and plow tillage (PT), were established in 2001 in Luancheng County, Hebei Province. The concentration, storage, and SR of SOC and soil total nitrogen (TN) were assessed in both the wheat and maize seasons. Compared with RT and PT, the mean SRs for all depth ratios of SOC under NT increased by 7.85% and 30.61% during the maize season, and by 14.67% and 30.91% during the wheat season, respectively. The SR of TN for 0–5:30–50 cm increased by 140%, 161%, and 161% in the maize season, and 266%, 154%, and 122% in the wheat season compared to the SR for 0–5:5–10 cm under NT, RT and PT, respectively. The data indicated that SOC and TN were both concentrated in the surface-soil layers (0–10 cm) under NT but were distributed relatively evenly through the soil profile under PT. Meanwhile, the storage of SOC and TN was higher under NT for the surface soil (0–10 cm) but was higher under PT for the deeper soil (30–50 cm). Furthermore, the storage of SOC and TN was significantly related to SR of SOC and TN along the whole soil profile (P2 would be uncommon under degraded conditions. Due to the differences in the soil-nutrient distribution by various management strategies, stratification of SOM (e.g., SOC and TN) has been reported as an important indicator of soil quality [2, 3, 20]. The index of SR allows kinds of soils to be compared on the same assessment scale because of an internal normalization procedure that accounts for inherent soil differences [2]. Furthermore, SR also emphasizes a surface beneficiation of SOM, which is vital to receive input nutrient, catch rainfall, partition flux of gases into and out of soil, more to soil quality and SOC sequestration capacity than just a high total standing storage of SOM [2–4, 13, 21]. A high SR of SOC could be an efficient indicator of the dynamic soil quality and SOC sequestration, regardless of the soil type and climatic regime [2, 3]. Previous studies have shown that the SR varied from 1.0 to 1.9 under conventional tillage and from 1.5 to 4.1 under NT [2, 3, 8, 22, 23], but no specific or consistent value of SR has been observed to indicate a high soil quality. As concluded by Franzluebbers [2], further research is need to enhance the usage of SR of SOM as a soil quality indicator.

PLOS ONE | DOI:10.1371/journal.pone.0128873 June 15, 2015

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Tillage Effects on Stratification and Storage of SOC and N

Tillage practices can affect the SR and ultimately soil quality due to a different distribution pattern of the soil properties. The adoption of CT can alter the soil properties by soil depth, e.g., SOC [24, 25], TN [11, 26], the enzyme activities [27], and the soil C:N ratio [22]. In contrast, plow tillage (PT) can cause a uniform distribution of SOM [2, 22]. Additionally, conversion from conventional tillage to NT can result in a redistribution in the SOC with soil depth and enhance the SOC sequestration [10, 28], particularly in the surface-soil layer [8, 10]. Therefore, soil quality is maintained under NT system by enhancing aggregation and facilitating aeration, mainly because of the enrichment of the surface SOC [8]. The North China Plain (NCP), with a predominant double cropping system of winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.), is one of the most important agricultural regions in China. However, several constraints exist that limit the agricultural development (e.g., water-resource scarcity, labor shortage, and low economic benefit) [18, 29]. Recently, the CT system as a C-smart practice has been adopted and popularized instead of the traditional system (e.g., PT) in this region due to its ability to enhance the SOC sequestration and its other benefits to the environment and crop production [28]. Du et al. [19] reported that NT enhanced the SR of SOC and TN in this region compared to that of PT; however, it did not increase the SOC and total N storage in the soil profile (0–50 cm). For some other previous studies conducted in this area also illustrated that CT could increase the surface SOC and TN content [30], enhance the net SOC sequestration rate [31], and improve the soil liable C pools [32]. Consequently, an assessment of the SR of SOM under various tillage practices is important to identify strategies for the sustainable management of soil resources in the NCP. Therefore, the objectives of this study were to investigate the stratification of SOC and TN, compare the differences in the SR of these soil properties for the wheat and maize seasons under various tillage systems, and assess the applicability of utilizing SR as an indicator of SOC sequestration and soil quality in the NCP.

Materials and Methods Ethics Statement This research was performed in cooperation with China Agricultural University and Luancheng Agro-Ecosystem Experimental Station. The farm operations of this experiment were similar to rural farmers’ operations and did not involve endangered or protected species. The experiment was approved by the Key Laboratory of Farming System, China Agricultural University and Luancheng Agro-Ecosystem Experimental Station.

Experimental site description The experiment was initiated in 2001 at the Luancheng Agro-Ecosystem Experimental Station (37°500 N, 114°400 E, elevation of this site is 50.1 m) of the Chinese Academy of Sciences, Hebei Province, in the NCP. The Experimental Station is located in piedmont of the Taihang Mountains. The annual average precipitation in this area is approximately 480.7 mm, 70% of which occurs during the summer, and the mean annual air temperature is 12.2°C. The predominant soil type is a silt loam with 13.8% sand, 66.3% silt, and 19.9% clay, 1.4 g cm-3 bulk density (ρb) in the plow layer and 9.11 g kg-1 of SOC in the top soil (0–10 cm). The principal soil properties for the 0–30 cm depth are listed in Table 1. The dominant cropping system in this region consists of winter wheat and summer maize. The growing season of the winter wheat is from midOctober to early June, whereas that of the summer maize is from mid-June to early October. Before the experiment, the tillage systems were moldboard plow tillage (PT) for winter wheat and no-till (NT) for summer maize in 1990s.


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Table 1. Principle soil properties before treatment. Soil depth (cm)

ρb (g cm-3)

TN (g kg-1)

AN (mg kg-1)

K (mg kg-1)

0–10

1.34

0.74

37.95

115

62.90

12.4

10–20

1.42

0.64

30.58

90

39.62

12.8

20–30

1.64

0.45

27.99

65

23.32

9.9

P (mg kg-1)

SOM (mg kg-1)

ρb, soil bulk density; TN, total nitrogen; AN, alkali-hydrolyzable nitrogen; K, available potassium; P, phosphorus; SOM, soil organic matter. doi:10.1371/journal.pone.0128873.t001

Experimental design The three tillage treatments included PT, RT and NT. Each tillage treatment was replicated three times, and the area of each plot was 560 m2 (8 x 70 m). The various tillage practices were implemented only in the winter wheat season, whereas direct seeding without any tillage was performed in the summer maize season for all of the treatments. The maize residues were chopped twice with a residue pulverizer for all treatments. The winter wheat was harvested by a combine harvester, and approximately 30 cm height of the wheat residue was retained in the fields in all of the treatments. The PT treatment was plowed once to a depth of 20 cm with a moldboard plow and then rotavated once to a depth of 8–10 cm before seeding. The RT treatment was rotavated twice to a depth of 8–10 cm. The residues were retained in the fields for all treatments as an average quantity of 6637, 6540, and 5966 kg ha-1 yr-1 for wheat and 9191, 9078, and 8573 kg ha-1 yr-1 for maize under PT, RT, and NT, respectively. The NT treatment involved seeding with a NT planter, which cut the residue, opened a small slot for seed placement, and applied fertilizer. The seeding rate was 150 kg ha-1 for the wheat and 50 kg ha-1 for the maize. Fertilizers for the winter wheat were applied at rates of 130 kg N ha-1 and 121 kg P ha-1 during sowing and another 138 kg N ha-1 at the regreening stage. Shortly after jointing, the summer maize was topdressed at the rate of 210 kg N ha-1. The winter wheat was irrigated with the pumping of groundwater for three times (at sowing, the green-turning stage and the jointing stage), at approximately 40–50 mm per irrigation. The summer maize was irrigated only when the rainfall was limited, and the amount of irrigation was the same as that for the winter wheat. To effectively control weeds and insects, herbicides and insecticides were applied one extra time under NT than under CT and RT during the winter wheat season.

Soil sampling and analysis Soil samples for SOC and TN were collected in triplicates in October (maize harvest time) of 2009 and June (wheat harvest time) of 2010. Soil bulk density was determined by the core method using a stainless steel ring (5 cm high and 5 cm in diameter) for 0–5, 5–10, 10–20, 20– 30, and 30–50 cm depths. Soil samples were oven dried at 105°C for 24 hr to obtain the dry weight [33]. The soil samples were obtained from the 0–5, 5–10, 10–20, 20–30, and 30–50 cm depths, and a composite sample from all replicates was obtained for each depth. The soil samples were air-dried, gently ground, and sieved (2 mm) in the laboratory. The concentration of SOC (g kg-1) was determined using the potassium dichromate oxidation titration method [34], and that of TN (g kg-1) was determined by the Kjeldahl method [35].

Stratification ratio calculation The SRs of SOC and TN were calculated by dividing the concentration determined for each soil property in the 0–5 cm layer by those in the 5–10, 10–20, 20–30 and 30–50 cm layers, following the procedure of [2].


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SOC and TN storage calculation The SOC and TN storage was calculated by the equivalent soil mass method. The equivalent soil mass SOC and TN storage were computed following Eq (1): " # n X Xn Melement ¼ ð1Þ Msoil;i conci þ Mj Msoil;i concextra 0:001 i¼1 i¼1

when i = 1, 2, 3, 4, and 5, this represents the 0–5, >5–10, >10–20, >20–30, and >30–50 cm soil depths, respectively. Melement (Mg ha-1) is the equivalent soil mass of the SOC and TN storage. conci is the concentration of SOC and TN in the soil depth. concextra is the extra SOC and TN concentration, and when i = 5, the concextra was assumed to be equal to the soil depth of 30–50 cm because SOC and TN changed little in the deeper soil.Mj is the certain soil mass, and when j = 1, 2, 3, 4, and 5, it represents the maximum soil mass under the various tillage treatments in the 0–5, 0–10, 0–20, 0–30, 0–50 cm soil depths. Msoil,i (Mg ha-1) was the soil mass of the soil depth, calculated according to Eq (2): Msoil; i ¼ rb;i Ti 10000

ð2Þ

where ρb (Mg m-3) is the soil bulk density, and Ti (m) is the thickness of the soil depth.

Data analysis The SPSS 16.0 analytical software package (SPSS Inc., Chicago, IL, US) was used for comparing the ANOVA. The concentration of SOC, TN and the SOC and TN storage were analyzed with ANOVA for the same depth under various tillage treatments and for the same tillage treatment under various soil depths. Differences among the treatments and soil depths were considered to be significant using the LSD test at P