1. Introduction
Carbon cycle is closely related to global energy balance and ecosystem productivity [
1], and the soil carbon pool is the largest in terrestrial ecosystems [
2]. Approximately 1500 to 2000 Pg (1 Pg = 10
15 g) of carbon is stored in soils in the form of organic carbon [
3,
4], accounting for more than half of the soil carbon pool worldwide; soil organic carbon (SOC) that exhibits active exchange with atmospheric components accounts for approximately two-thirds of the terrestrial ecosystem carbon [
5]. The organic carbon in the soil is a carbon sink and source, and its small changes greatly alleviate or accelerate the concentration of atmospheric CO
2, thereby changing the global carbon cycle [
6]. Soil carbon is the balance between carbon input into and loss from the soil. In the wake of global warming, especially in recent decades, the existing dynamic equilibrium between soil and atmospheric pools has been shifted by climate and human intervention, and a large amount of SOC has been oxidized and released into the atmosphere in the form of CO
2 due to the long-term and large-scale global farming, thereby resulting in high emissions of greenhouse gases [
7]. Land use change caused by human activities, including farming, is a main driver to terrestrial ecosystem carbon storage. Approximately 10% of the world’s land is used for agricultural production [
8]. Farmland soil carbon pools are more active and susceptible to human activities than other soils, but they can also simultaneously self-adjust in a short time [
9]. In recent years, researchers in China and abroad have paid considerable attention to the role of farmland soil in carbon fixation. In regions with high agronomical pressure, carbon management has a remarkable influence on crop productivity and yield [
10]. Farmland SOC pool, an important part of the soil carbon pool, can affect farmland productivity by influencing soil quality and global carbon cycle by affecting atmospheric CO
2 concentration [
11]. Therefore, SOC and its components in farmland must be investigated and analyzed to achieve global ecosystem carbon balance and sustainable utilization of farmland soil.
Paddy field area accounts for 23% of the total cultivated land in China and 20% of the total rice planting area in the world [
12]. Winter warming, a result of greenhouse effect, has been observed recently in southern China, where 90% of paddy soils in China are distributed [
13]. Paddy soil exhibits a high level of organic carbon content, an evident trend of carbon sequestration, and a great potential for carbon fixation, which is an important part of the construction of soil carbon sequestration capacity to reduce greenhouse gas emission. In recent years, most studies of SOC composition in paddy fields focused on the impact of land use mode, farming practices, or fertilization on its content but less on water management and biochar application [
14,
15,
16]. Related research only focused on the effects of single factors, such as irrigation or biochar, on SOC [
13,
17]. Previous studies have shown that frequent wet–dry alternation in water-saving irrigation paddy fields promotes soil respiration [
18] and remarkably stimulates CO
2 release from soils [
19,
20]. The application of water-saving irrigation reduces the SOC content in paddy fields. Water-saving irrigation should be combined with farmland carbon management measures, such as straw returning, organic fertilizer application, and biochar application, to realize the sustainable utilization of water and carbon resources in paddy fields.
Biochar is a by-product of the residual carbonization of biomass organic carbon after being separated from combustible gases under anaerobic or hypoxic conditions and high-temperature pyrolysis. This by-product presents a high degree of aromatic ring molecular structure and porous characteristics [
21] and higher thermal stability and stronger adsorbability than general organic materials. Biochar application is considered a potential effective measure to increase carbon retention in soils and mitigate the effects of greenhouse gas [
22,
23]. Many studies have shown that the application of biochar can rapidly enhance soil carbon pool [
24]. Applying biochar has beneficial effects on soil active organic carbon components, such as an increase in crop biomass and input of fresh organic carbon [
25], improvement in soil structure, promotion of soil aggregate formation [
26], and provision of an ideal habitat for soil microorganisms [
27]. Biochar is suggested to improve soil quality, and there are reports of stimulated microbial response and loss of native SOC [
28]. In addition, the effectiveness of biochar depends on its raw material and production conditions, quality and application rates, and type of soil [
29]. However, whether the application of biochar in water-saving irrigation paddy fields manifests the same effect remains to be verified. With the development of rice production in China, rice water-saving irrigation technology has been widely applied to achieve high-efficiency, high-yield, energy-saving, and environmental protection. After the application of water-saving irrigation technology, the paddy field is in an unsaturated state in certain or most rice-growing stages, and the change of soil moisture is bound to affect the physical and chemical properties of soil. Accordingly, the transformation of biochar in the soil changes, but no relevant research is available yet. Studying the mechanism of biochar application on SOC and its components in water-saving irrigation paddy fields is important to cope with the shortage of water resources and reduction of SOC in water-saving irrigation paddy fields [
30].
This study investigated the temporal and spatial distributions of SOC, water-soluble organic carbon (WSOC), and soil microbial biomass carbon (SMBC) in paddy fields under different irrigation methods and biochar application rates. The responses of catalase (CAT) and invertase (INV) to biochar application in paddy fields under water-saving irrigation were also analyzed. Apart from exploring the conjunct influences of rice water-saving irrigation technique and biochar application on SOC composition and related enzyme activities, this study also aimed to enrich the theory of water-saving irrigation and propose a reasonable water-carbon regulation model to provide a scientific basis for the sustainable utilization of soil and water resources in paddy fields.
2. Materials and Methods
2.1. Experimental Site
The experiment was conducted in 2017 at the State Key Laboratory of Hydrology-water Resources and Hydraulic Engineering of Hohai University, Kunshan Experiment Station (34°15′21″ N and 121°05′22″ E), located in the Eastern Taihu Lake region, China. The study area is part of the subtropical monsoon climate zone in Southeast China, with a mean annual air temperature, precipitation, evaporation, and sunshine duration of 15.5 °C, 1097.1 mm, 1375.9 mm, and 2104.9 h, respectively, and frost-free period of 232 days/year. The locals are accustomed to rice-wheat rotation. Soil in the experimental site is classified as dark-yellow hydromorphic paddy soil. The basic properties of this soil classification are as follows: organic matter of 21.71 g/kg for the top 0 to 18 cm layer, total N of 1.79 g/kg, total P of 1.4 g/kg, total K of 20.86 g/kg, pH of 7.4, and soil bulk density of 1.32 g/cm3 for the 0 to 30 cm layer.
2.2. Experimental Design
The experiment was laid out (pot size 80 cm × 50 cm × 70 cm) in a randomized block design with four treatments and three replicates. The four treatments were a combination of irrigation and rice straw biochar addition, and the two irrigation managements were controlled irrigation (CI) and flooding irrigation (FI). The three biochar managements under CI conditions were 0, 20, and 40 t/ha, and only one biochar application level (40 t/ha) was set under FI conditions. The four treatments were C0 (CI and 0 t/ha), C20 (CI and 20 t/ha), C40 (CI and 40 t/ha), and F40 (FI and 40 t/ha). The pots were individually constructed containers, which had a drainpipe buried at the bottom of the pot. The drainage can be manually controlled by the drainpipe. The soil substrate was taken from the local natural soil and was loaded and compacted layer-by-layer according to the order of soil sampling to ensure that the soil layer in the pots was consistent with the natural soil. All treatments were applied to the same pots during the experiment. The variety of rice planted in this experiment was Suxiangjing rice. Three to four seedlings per hill were transplanted with 13.0 × 25.0 cm hill spacing on 30 June 2017 and harvested on 31 October of the same year.
The rice straw biochar used in the experiment was provided by Zhejiang Biochar Engineering Technology Research Center. The biochar was spread in the pots manually and incorporated into soil (approximately 20 cm) using a shovel 1 day prior to transplantation of rice in 2016. The main properties of biochar were as follows [
31]: C content of 42.6%, total N of 0.75%, total P of 0.15%, total K of 1.06%, special surface area of 81.9 m
2/g, total pore volume of 0.08 cm
3/g, and pH of 10.1. For the FI treatment, a standing water depth of 3 to 5 cm was maintained after transplantation, except for the late tillering and yellow maturity stages. For the CI treatment, the ponded water depth was only kept at 10 to 30 mm in the regreening stage. Irrigation was applied to maintain soil moisture, and standing water was avoided in other stages, except during periods of pesticide and fertilizer applications, with a combination of soil moisture for root layer accounting for 60% to 80% of saturated soil moisture content as an irrigation control indicator [
32]. A conventional fertilizer application was managed in accordance with local farmers’ customary fertilization methods and fertilization amount, such as 328.48, 45.00, and 63.75 kg/hm
2 of N, P, and K fertilizers, respectively.
2.3. Soil Sampling and Analysis
Rice-wheat rotation was carried out in the Taihu Lake region in China. Starting from the end of June when rice was transplanted and ending in October when rice was harvested, the rice growth period lasted for approximately 125 days. Soil samples were collected four times before transplantation (27 June), at the tillering (30 July), jointing and booting (28 August), and milk stages (29 September). Soil samples of 0 to 10, 10 to 20, and 20 to 40 cm were collected, using the “S-shape” method in each pot. After the visible plant residues and gravels were removed, the soil samples were divided into two parts as follows: one was fresh soil samples stored in a refrigerator at 4 °C to determine SMBC and WSOC, and the other was naturally air-dried and passed through 20- and 100-mesh sieves to determine soil enzyme activity and total SOC.
Specific determination methods: SOC was determined by K dichromate oxidation [
33]. WSOC was determined using the K dichromate-concentrated sulfuric acid external heat capacity method (wet oxidation method) [
34]. In a study by Wu et al. [
35], SMBC was determined using the fumigation extraction method of chloroform. The soil INV activity was determined by 3,5-dinitrosalicylic acid colorimetry. Soil CAT activity was determined by K permanganate titration. The water layer was recorded at 8:00 by using vertical rulers, which were pre-embedded in the pot. Soil moisture was measured with a portable time domain reflectometer when no water layer existed. A 1 L measuring cup was used for irrigation, and the amount of irrigation was recorded in time.
2.4. Statistical Analysis
Excel 2010 was used to initially analyze data and create a database for drawing related charts. Statistical analyses were performed using standard procedures for a randomized plot design (SPSS 22.0, SPSS Inc., Chicago, IL, USA). Significance was calculated on the basis of F-tests and least significant differences at 0.05 probability level. Data principal component analysis (PCA) and mapping were completed by using Origin 2017.