Effect of different biochar addition rates on Soil Physical Properties in rain-fed farmland

: Biochar is extensively used for ameliorating soil structure. However, the evaluation of soil physical properties influenced by its large-scale use is still unclear. Moreover, research on biochar is mainly based on laboratory tests, lacking medium and long-term field positioning tests. In this study, a local field trial was conducted for 3 years on the Loess Plateau in central Gansu, China. Six levels of biochar treatment were set with application amounts of 0 t·ha -1 , 10 t· ha -1 , 20 t·ha -1 , 30 t·ha -1 , 40 t·ha -1 , and 50 t·ha -1 (CK, T1, T2, T3, T4, and T5, respectively). The influence of various input levels of biochar on soil porosity, bulk density, aggregate stability, and soil saturated hydraulic conductivity from the 0 cm to 30 cm layer were studied during the three years of wheat field experiments. The experimental study indicates that the increase of biochar addition can significantly (p < 0.05) reduce soil bulk density and increase not only soil saturated hydraulic conductivity but also total soil porosity, content of soil mechanically stable aggregates, and its MWD from the 0 cm to 30 cm soil layer. Among them, T5 treatment is the most significant (p < 0.05). While its content determination of Water Stable aggregates and its MWD only have an obvious (p < 0.05) improvement effect from the 0 cm to 20 cm soil layer, and T5 is the most significant. Therefore, large-scale biochar application is conducive to the improvement of farmland soil physical properties in this region.


Introduction
Biochar, an extremely aromatic and insoluble solid material, is generated by pyrolysis and carbonization of crop straw under conditions of complete or partial hypoxia 1 . It has become a new research hotspot to ameliorate soil fertility and the ecological environment in recent years. The production and introduction of biochar in soil is a new process to form a standing carbon dioxide (CO 2 ) storage sink, with alleviate the increase of atmospheric CO 2 caused by carbon storage 2 , and its application can ameliorate soil physical properties and crop yield 3 .
A lot of research 411 on biochar in increasing soil carbon sequestration, improving soil physical properties, increasing crop yield and reducing greenhouse gas emissions has been conducted. But the current conclusions are not consistent. Some studies 1213 proved that the introduction of biochar can strengthen the stability of soil aggregates and enhance the content determination of Water Stable aggregates in soil. Other studies 1519 pointed out that the introduction of biochar did not make obvious impact on the stability and content determination of Water Stable aggregates in soil. After applying biochar for 3 years, Chen et al. (2011) pointed out that biochar could only obviously decrease the bulk density of soil surface(0mm-75mm), other results indicated that biochar could decrease soil bulk density in 0-30cm soil layer 11 . Yet, the biochar-amended soil indicated an enhance in soil total porosity and an obvious reduce in bulk density 20 . Oguntunde et al. (2008) also showed that biochar has a good function on soil total porosity and the ability of soil to keep water, but if overapplied, biochar may clog soil creases and even affect soil water permeability.
The semi-arid region on the Loess Plateau in the middle of Gansu Province has become one of the region which has most worst ecological environment and serious soil erosion in China and even in the world. And these results are due to its topography, climate, soil structure and other factors 2122 . Some studies showed that increasing the introduction of biochar in agricultural soils reduces the friability index of aggregates in soil and improves the properties of aggregates in soil, thereby minimizing soil erosion potential 23 . In addition, biochar has not been widely used in China and is still in the experimental research stage. The research mainly focused on short-term laboratory test, and previous studies focused more on the effect of a smaller amount of biochar addition level (0.3-2%, w / W) 2325 , while the impact assessment of a large amount of biochar addition was very scarce. Therefore, it is urgent to study the impact of the appropriate amount and the larger addition amount of biochar about soil structure, chemical and physical properties in the semi-arid Loess Plateau.
In this study, we measured and analyzed the total porosity, bulk density, aggregate content and stability of biochar application at different levels after three years to dry farmland. The ultimate goal is to find the right amount of biochar suitable for farmland soil in this region and provide reliable basis for related research.

Description of Study Region
A sandy-loam soil was selected as the research object near Dingxi of China (104°44'E, 35°28'N),which is a typical rain-fed agricultural area. The average temperature in this area is 6.4 degrees Celsius and the average rainfall is 390.9 millimeters each year. And it belongs to a semi-arid climate.
The study field was separated into 18 plots (2.8 × 6 m) receiving six different doses of biochar. The amounts of biochar were respectively from 0 to 50 t ha−1 (Table 1). A randomized block design was the most appropriate method for this study, and the experiment would be repeated three times. Biochar was applied once before rotary tillage in March 2015, and an accompanying mineral NPK fertilizer was added to each treatment. The biochar which is obtained from Golden Future Agriculture Technology (Liaoning, China). And it is formed by the pyrolysis of maize straw at a high temperature of 500°C. It contained 53.28% total carbon (C), 0.26% total P and 1.04% total N. The wheat used for experiments("Dingxi 35") was planted on the field plot in 2015. We can found more information about the study site and the biochar in Jiang et al. 26 .

Soil collection and analyses
After harvest in July 2017, about 1000 g of soil samples of 0-5cm, 5-10cm and 10-30 cm soil layers in every site were carried out according to randomly selecting five soil cores. Undisturbed soil samples would be professionally air-dried in specific lab, and that was used to test and analyze the mechanical stability, water-stable aggregate content, composition and stability. Afterward, bulk density and total porosity were using ring knife method 26 , and Saturated hydraulic conductivity of soil was carried out by using a permeameter method 28 . Soil aggregates: after obtaining the aggregate contents of ≥ 5, 2 ~ 5, 0.25 ~ 2 and < 0.25 mm by dry sieve method and Yodel wet sieve method 26 , the aggregate stability is characterized by the aggregate content of ≥ 0.25 mm, and average weight diameter MWD 26 , as follows (Eq.(1)): In the formula (Eq.(1)) : R is the average diameter (mm) of a certain level of aggregate, and w is the dry weight of the aggregate (kg) of this level .

Statistical analysis
In this study, some professional analytical methods will be introduced. For example, the SPSS version 25.0 (SPSS Incorporated, USA), single-factor analysis of variance (P <0.05) and Fisher's Least Significant Difference test.

Soil bulk density(SBD) and total porosity
The SBD and total porosity are indicators to measure soil compactness.The SBD of each treatment increased with the deepening of soil layer, and the total porosity was on the contrary. Under different biochar application rates, the SBD was the highest in CK treatment and the lowest in T5 treatment. Contrasted CK with the SBD, the SBD of T1, T2,T3, T4 and T5 was strikingly decreased (P <0.05), by 5.38, 5.80, 7.99, 9.10 and 12.24% (0-5 cm); and by 7.97, 9.86, 9.78, 10.28 and 14.98% (5-10 cm), and by 9.16、10.98、10.69、11.31 and 11.31% (10-30 cm), respectively ( Table 2).
Different letters point out obvious differences among the experiments (LSD, P < 0.05).  Figure 1 shows that saturated hydraulic conductivity in soil was the worst in CK treatment and was the best in T5 treatment. The total porosity of T1, T2, T3, T4 and T5 strikingly improved (P <0.05) by 12.02, 17.62, 22.97, 26.38 and 35.46%, compared with CK.

Soil Water Stable aggregates
Under different treatments, Water Stable aggregates in the MWD under virous experiment decreased with the deepening of soil layer (Fig. 2c) 5-10 cm). With the increase of content, the increase of by 10-30cm was not significant. With the increase of biochar amounts, the improvement of MWD was less obvious at 10-30cm soil layer (Fig. 2c). With the strength of biochar content, the MWD of soil Water Stable aggregates from 10 to 30cm soil layer did not change distinctly (Fig. 2c).
The diameter of Water Stable aggregates is≥5mm, 0.25-2 mm and 2-5mm decreased with the deepening of soil layer, while the non Water Stable aggregates (< 0.25 mm) were on the contrary (Fig. 2d). And the content determination of non Water Stable aggregates (< 0.25 mm) was the highest in all treatments, and the content determination of Water Stable aggregates increased with the decrease of particle size (0-30 cm) (Fig. 2c). The content determination of Water Stable aggregates about various particle sizes was the lowest in the treatment of CK (0-30cm), the highest in T5 and the second in T4 in 0cm-5cm and 10cm-30 cm soil layer, and the highest in T4 and the second in T5 in 5-10cm soil layer. The content determination of non Water Stable aggregates were on the contrary (Fig. 2d). The amount of Water Stable aggregates with granularity of 0.25mm, 0.25-2mm, 2-5mm and 5mm in the total Water Stable aggregates varies from 66.35-82.15%, 8.61-15.61%, 5.34-10.47% and 3.58-7.90% respectively.

Discussion
Biochar strinkingly have a positive function in physical and chemical properties of soil, especially in physical properties [10,22 . Since biochar is produced from pyrolysis, it has a great specific surface area and a loose structure 1 . In addition, its unit volume mass is low. After being applied to soil, the SBD can be quickly decreased and the total soil porosity can be increased 7 . The final data from this study shows that biochar has obvious impacts on decreasing the SBD and increasing the total porosity of topsoil (0-30cm), especially in 0-10cm soil layer, its effect is better than that in 10-30cm soil layer. However, some other studies 29 found that biochar had a more striking effect on the deep soil, and the SBD from 20cm to 30cm soil layer reduced significantly, which may attribute to the introduction of Tillage in this experiment. Tillage (tillage depth is about 18 cm) can evenly mix biochar into the topsoil soil, and may also be related to some factors, such as the amount of biochar added and soil type. Our results mean that introduction of biochar improved the saturated hydraulic conductivity of soil,which is highly correlated with the content determination of soil organic matter and non capillary porosity, while the biochar itself has the characteristics of loose and porous, and it is also correlated with the increase of soil organic carbon content by the addition of biochar 26 .
As an important cementation material of soil aggregates, organic carbon can bond more clay and silt particles together then convert them into aggregates, which can strikingly ameliorate the content and stability of aggregates 22 . Meanwhile, biochar not only has a large specific surface area, but also contains organic macromolecules. At the same time, the introduction of biochar improves microbial activity, increases the content determination of organic secretions, and then promotes the agglomeration of soil particles. However, some studies 31 indicated that biochar can be adsorbed on the surface of clay particles through hydrogen bond, intermolecular Van der Waals force and surface Coulomb force, so as to hinder the direct cementation between clay particles, and does not appear to be having a strinking impact on the size and content determination of soil aggregates. This may be related to soil type, study area and content of biochar addition. This study also indicated that biochar has a great function about the growth of the stability of mechanical stability aggregates and water stability aggregates in 0-30 cm soil layers. The reason may be that the addition of biochar can cushion the damage of external forces to the soil structure stabilities, so that increase the content determination of mechanically stable aggregates.
This experiment has only been carried out for 3 years. In order to evaluate the impact of straw and biomass carbon about the stabilities of soil aggregates more accurately and objectively , the indexes of soil aggregates in different years should also be observed. In addition, on June 20 and July 14, 2017, Lijiabu Town, Dingxi city suffered a severe hail disaster, which seriously affected the growth of wheat and led to no yield. Therefore, this study failed to quantitatively measure the function of biomass carbon addition on crop yield. However, other experiments carried out in this study area show that biochar can improve wheat yield 11 , which may because of the improvement of soil physical properties.

Conclusion
The present results indicate that the addition of biochar can significantly reduce soil bulk density (SBD) from the 0 cm to 30 cm soil layers, and the T5 treatment has the most significant effect on SBD reduction. Additionally, the incorporation of biochar can improve soil total porosity and soil saturated hydraulic conductivity, with the T5 treatment showing the most noticeable effect. With the exception of the T1 treatment, all other treatments can significantly (P≤5%) ameliorate the content determination of soil mechanically stable aggregates and its MWD in each soil layer (0cm-30 cm), and the T5 treatment has the most effective improvement. While the content determination of Water Stable aggregates and its MWD only improves significantly(P≤5%) from the 0cm to 20 cm soil layer, and T5 shows the highest improvement. Therefore, the addition of biochar is conducive to the improvement of farmland soil physical properties in this area, promoting crop yield and sustainable development of agriculture in this region.