Combined application of biochar with nitrogen fertilizer improves soil quality and reduces soil respiration whilst sustaining wheat grain yield in a semiarid environment

This study was conducted to investigate the effect of biochar, straw and N fertilizer on soil properties, soil respiration and grain yield of spring wheat (Triticum aestivum L.) in semi-arid Western Loess Plateau of northwestern China. The two carbon sources (straw and biochar) were applied alone or combined with nitrogen fertilizer (urea, 46% nitrogen [N]), whilst the soil without carbon is made up of nitrogen fertilizer applied at 0, 50 and 100 kg N/ha. The experiment was arranged in a randomized complete block design with three replicates and was conducted in 2014, 2015 and 2016 cropping season. Results showed that the greatest grain yields were found with 100 kg N ha−1 fertilization rate under biochar, straw and soils without carbon, but the greatest effect occurred on the biochar amended soils. Biochar amendment produced the greatest grain yield at 1906 kg ha−1, followed by straw treated soils at 1643 kg ha−1, and soils without carbon the lowest at 1553 kg ha−1. This results is supported by the fact that, biochar amended soils (at 0–10 cm) increased soil organic C by 17.14% and 21.65% compared to straw treated soils and soils without carbon respectively. Seasonal soil respirations were between 19.05% and 23.67% lower in BN100 compared with SN50 and CN100. Soil respiration reduced with increasing N fertilization rates under all treatments, but the greatest effect occurred on biochar plots. Biochar amended soils decreased carbon emission by 26.80% and 9.54% compared to straw treated soils and soils without carbon amendment respectively. Increased grain yield and the decreased carbon emission in BN100 translated into greater carbon emission efficiency (2.88 kg kg−1) which was significantly different compared with the other treatments. Combined application of biochar with 100 kg N ha−1 in rainfed spring wheat was a suitable agricultural practice.

were found with 100 kg N ha -1 fertilization rate under biochar, straw and soils without carbon, 23 but the greatest effect occurred on the biochar amended soils. Biochar amendment produced the 24 greatest grain yield at 1906 kg ha -1 , followed by straw treated soils at 1643 kg ha -1 , and soils 25 without carbon the lowest at 1553 kg ha -1 . This results is supported by the fact that, biochar 26 amended soils (at 0-10 cm) increased soil organic C by 17.14% and 21.65% compared to straw 27 treated soils and soils without carbon respectively. Seasonal soil respirations were between 28 19.05% and 23.67% lower in BN 100 compared with SN 50 and CN 100 . Soil respiration reduced with 29 increasing N fertilization rates under all treatments, but the greatest effect occurred on biochar 30 plots. Biochar amended soils decreased carbon emission by 26.80% and 9.54% compared to 31 straw treated soils and soils without carbon amendment respectively. Increased grain yield and 32 the decreased carbon emission in BN 100 translated into greater carbon emission efficiency (2.88 Introduction 38 Nitrogen fertilization and crop residue retention are known to influence soil organic C dynamics be explained by the fact that increased SOC resulting from increased crop biomass returned to forms of C, which may also increase the risk of losses through leaching [7]. Soil incorporation of 48 crop residues, particularly with high C:N ratio, contributes to maintain soil organic C levels, 49 enhance biological activity, and increase nutrient availability [8]. Increasing SOM is a slow 50 process, particularly in semiarid environments due to naturally low biomass-C inputs [6]. 51 Recent increases in carbon dioxide emissions levels require that management practices 52 conducive to improved soil C sequestration should be undertaken [9]. Soil carbon sequestration 53 through application of recalcitrant C-rich biochar is mentioned as a suitable means to mitigate 54 climate change, improve soil fertility [4], and crop productivity [5]. Biochar is a recalcitrant 55 source of C, which following soil application contributes to slow down the turnover of native 56 SOC [10]. However, these effects have vary significantly depending upon the type of biochar 57 used and the soil conditions under which the material is applied.

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The Loess Plateau is considered the cradle of agricultural cultivation in China and is 59 widely used for grain production. Yet little attention has been given to developing agro-system 60 specific strategies to soil degradation from these regions. This region has experienced a 61 progressive decline in crop productivity because of soil degradation processes. Progressive loss 62 of soil organic matter, associated with traditional methods of soil cultivation often accelerates 63 soil erosion processes, the decline of soil fertility, and loss of soil organic C [11]. Several studies 64 have been conducted to quantify CO 2 emissions from loess soils in North West (NW) China [12], 65 but there appears to be limited information available about the specific impact of widely-used 87 2240°C and annual radiation is 5930 MJ m -2 , with 2477 h of sunshine. These conditions are 88 representative of those commonly found within agricultural areas of semiarid environments.

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Prior to the experiment, the site had been occupied by potatoes (Solanum tuberosum L.).  after threshing and spread evenly on the soil surface. Biochar and straw were applied at the same 106 quantity based on the straw returned to the soil every year. All the treatments received a blanket 107 application of phosphorus fertilizer which was applied at 45.9 kg ha -1 P as ammonium and elemental analysis using atomic adsorption spectroscopy. The Brunauer-Emmett-Teller 126 (BET) method was used to determine biochar surface area. Straw samples were oven-dried at 127 70°C for 72 h, and milled to pass through a 1-mm sieve. Total C and N contents of the straw, ash 128 content and pH were determined using the same procedure described for biochar. Table 1 shows 129 the chemical characterization of biochar and straw used in the experiment.  Concurrently with soil respiration measurement, soil temperatures were measured at 5, 10, and 135 15 cm deep using a geo-thermometer inserted into the soil near the chambers. Soil moisture at 136 the 0-5 and 5-10 cm depth intervals was determined by taking a soil core of 5-cm in diameter, 137 and subsequently drying the soil at 105°C for 24 h each time gas fluxes were measured.

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Gravimetric water content at the three depth intervals was multiplied by soil bulk density to 139 obtain the volumetric water content, which is expressed in cm 3 cm -3 . Soil samples were collected  μmol CO 2 /m 2 /s to g CO 2 /m 2 /h, 0.2727 converted g CO 2 /m 2 /h to g C/m 2 /h, and 24 and 10 were to 167 convert g C/m 2 /h to kg/ha for the growing season. To quantify grain yield per unit of carbon emission, carbon emission efficiency (CEE) which 172 was expressed as follows [23] 173 CEE = grain yield (kg /ha) carbon emission (kg /ha)

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Grain yield 175 Plots were hand-harvested using sickles to a height of 5-cm above-the-ground and by discarding  Differences between-treatments means were determined using Tukey's honestly significant (HSD) 182 difference test. Significance were determined using a probability level of 5%.   (Table 3). There was a significant interaction (p<0.05) between C and year for SOC, except at 224 the 10-30 cm depth interval (Table 4). Both carbon and fertilizer-N also had a significant effect  (Table 4), which corresponded in most cases to significant difference. However, the effect of N 50 228 was lesser in many instances relative to N 100 . Irrespective of N level, the soil organic carbon was 229 the lowest with no carbon, followed by straw and then biochar treated soils. Application of N at 230 50 and 100 kg ha -1 , increased microbial biomass carbon (MBC) significantly (p< 0.05) in all 231 treatments, but the greatest effect was recorded on straw treated plots, followed closely by the 232 biochar amended soils (Table 5).

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The significance of retaining crop residues was emphasized in this study by the 234 difference of soil C fractions between the organic amendment soils and the no carbon soils. The 235 greater soil C fractions found in the straw and biochar amended soils supports the hypothesis that increased C inputs results in improved soil carbon stocks. In this study, the increased C content 237 in biochar could be related to its high C content and the fact that biochar could slow down       The seasonal soil respiration over the study period had similar peak times and patterns for all the 271 treatments (Figs 3a-c). The seasonal Rs showed more emission from the straw-mulched 272 treatments in 2014 (Fig 3a) and 2015 (Fig 3b) than from the biochar treated plots and no carbon 273 soils. The major production peaks was observed during the peak growing season of the crop. showed an interaction effect on carbon emission (Table 6). BN 100 treatment decreased carbon 278 emissions by 12.37% and 18.80% compared to CN 0 and CN 50 soils, respectively (Table 7).      showed significantly lower carbon emission, but the effect of BN 100 was consistently greater.

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Reduced overall emissions and increased yield translated into higher carbon emission efficiency.

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The results suggest biochar + N-fertilizer nutrient management approach can simultaneously 373 manage soil health, improve yields while minimizing potential negative impacts of agricultural 374 activities on the environment.