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Article

Linkage of Crop Productivity to Soil Nitrogen Dynamics under Biochar Addition: A Meta-Analysis across Field Studies

by
Leiyi Zhang
1,2,†,
Meixia Zhang
1,†,
Yantao Li
3,
Jianling Li
1,
Yiming Jing
4,
Yangzhou Xiang
5,
Bin Yao
6 and
Qi Deng
1,*
1
Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China
2
Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510006, China
3
School of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
4
Key Laboratory of Bio-Resources and Eco-Environment of the Ministry of Education, School of Life Sciences, Sichuan University, Chengdu 610065, China
5
School of Geography and Resources, Guizhou Education University, Guiyang 550003, China
6
Institute of Desertification Studies, Chinese Academy of Forestry, Beijing 100091, China
*
Author to whom correspondence should be addressed.
†
Leiyi Zhang and Meixia Zhang contributed equally to this work.
Agronomy 2022, 12(2), 247; https://doi.org/10.3390/agronomy12020247
Submission received: 8 December 2021 / Revised: 28 December 2021 / Accepted: 13 January 2022 / Published: 19 January 2022
(This article belongs to the Section Soil and Plant Nutrition)
Browse Figures
Review Reports Versions Notes

Abstract

:
Biochar addition is a promising solution to improve soil nitrogen (N) availability and enhance crop productivity. However, a comprehensive understanding of the impact of soil N dynamics on crop productivity under biochar addition remains elusive. Here, we conducted a meta-analysis from 93 peer-reviewed field experiments to synthesize the linkage of crop productivity to soil N dynamics under biochar addition. The results show that biochar addition significantly enhanced crop productivity by 12.8% and plant N uptake (PNU) by 22.6%, and there was a strongly positive relationship between crop productivity and PNU. Biochar addition also significantly increased the contents of soil total N (TN), inorganic N (IN), microbial biomass N (MBN), and biological N2 fixation (BNF) by 3.34–18.7%, but reduced nitrous oxide (N2O) emission by 15.9%. Further analysis with the aggregated boosted tree model indicated that the TN and BNF played the most important roles in biochar-induced change in crop productivity. However, while crop productivity was positively correlated with TN under biochar addition, it showed a negative correlation with BNF. These findings suggest that biochar addition could enhance crop growth and productivity through increasing the soil N resource and N uptake, but this was not true for some N2-fixing crops, probably because they were usually constrained by nutrients other than N.

1. Introduction

Increasing crop productivity in the agroecological system is conducive to (1) increase the utilization of carbon dioxide and mitigate global climate change [1], (2) to improve the biodiversity and functioning of ecosystems [2], and (3) to provide enough food for the global growing population [3]. Nitrogen (N) is a key nutrient limiting crop productivity [4]. Improving N availability and use efficiency is thought to effectively enhance crop growth and productivity [5,6]. However, it is still difficult to directly quantify N availability as it is regulated by multiple processes of N dynamics in the soil, including N pool size and flux, N uptake, N fixation, and N loss [7,8]. Therefore, a better understanding of how soil N dynamics contribute to crop productivity has important implications for alleviating the global food crisis and climate change.
The application of biochar into the soil is considered as a promising solution to improve soil N availability and enhance crop productivity [9,10]. For example, previous studies have indicated that biochar addition could improve N use efficiency and increased N uptake in crop by reducing soil N2O emissions and N leaching, thus enhancing crop productivity [11,12]. However, some studies also reported that biochar addition did not change crop N uptake and productivity, despite increasing soil total N content and greatly reducing soil N2O emissions and N leaching [13,14,15]. Several reviews and meta-analyses have been conducted to comprehensively understand how biochar addition impacts soil N dynamics and crop productivity, respectively [9,10,16,17,18,19]. To the best of our knowledge, the linkage of crop productivity to soil N dynamics under biochar addition has not been well characterized on a global scale [20].
The reliability of meta-analysis results is highly dependent on the quantity and quality of data used for the analyses [21]. However, data used in the previous meta-analyses were mainly from the composite data of laboratory and field experiments, with less of the data being from field experiments [9,19,22]. Under laboratory conditions, biochar addition may exhibit several positive effects on soil N properties because of high application rates and homogenous biochar incorporation, and the results may be different from those observed under field conditions [18,23]. For example, previous reviews and meta-analyses found contrasting changes in soil inorganic N between laboratory and field experiments [17,24], and a significantly lower potential for mitigating N2O emissions and promoting crop productivity in field rather than laboratory experiments [9,25]. With the rapid increase of biochar experiments in the related field studies in recent years [26,27,28], it is necessary to further synthetic analysis and reveal the authentic mechanisms of how soil N dynamics influence crop productivity under biochar addition based on the large data set of observed results from the robust global field studies. This will help biochar systems to become attractive to ecosystem managers, who guide the practical application of biochar and N fertilizer to improve crop biomass and productivity and mitigate climate changes globally.
Herein, the meta-analysis using data collected from 93 peer-reviewed field experiments from 1990 to October 2020 should be more favorable to comprehensively analyze and understand how soil N dynamics influence crop productivity under biochar addition on a global scale. From the meta-analysis, the objectives of this study were (1) to quantify the effects of biochar addition on crop productivity in the global field conditions, (2) to determine the relationship between crop productivity and N uptake under biochar addition, and (3) to identify the major control factor of soil N dynamics in regulating crop productivity under biochar addition.

2. Materials and Methods

2.1. Data Sources and Compilation

The published articles were searched on well-known and comprehensive SCI (science citation index) (Note S1) searching libraries (i.e., Web of Science and Google Scholar) by using the following keywords: “biochar OR black carbon AND nitrogen OR N OR nitrate OR ammonium OR mineral N AND crop OR crop yield OR crop productivity AND soil AND field”. Articles were selected for this meta-analysis if they satisfied the following criteria: (1) only field experiments were included; (2) post-physiochemically modified biochars, which are produced by anaerobically pyrolyzing organic materials, were not considered; (3) results of N dynamics and crop productivity were concurrently reported in each paper; (4) each treatment included at least three replicates; (5) the control and biochar treatments were subjected to the same management practices (i.e., tillage, irrigation, fertilization, and residue additions), and the only difference between the control and biochar treatments as the application of biochar in the latter treatments; and (6) the original data could be extracted from tables and/or figures of the manuscript, including the mean and standard deviation (SD) or standard error (SE) (the SE was calculated by SD = SE × √n, where n is the replicate number). Approximately 13.98% (13 out of 93) of the observations failed to report any information on the variance (SD or SE), but one-tenth of the value of the mean could be used to assign the SD in such cases [29]. The spatial distribution of the matched studies is mapped in Figure 1. Relevant information about the data is included in Appendices A and B of the Supplementary Materials.
This study focused on evaluating the effects of soil N dynamics on crop productivity under biochar addition, including crop productivity (i.e., crop aboveground biomass (CAB), belowground biomass (CBB), and crop yield), in the N dynamic processes including plant N uptake (PNU), soil N pools (i.e., soil total N (TN), microbial biomass N (MBN), ammonium N (NH4+-N) and nitrate N (NO3−-N), inorganic N (IN: If there were data on the NH4+-N and NO3−-N, but not on the IN, we used the sum of the NH4+-N and NO3−-N contents to represent the mean of the IN, and the SDIN = SD 1 2 + SD 2 2 , where SD1 and SD2 are the SD of NH4+-N and NO3−-N, respectively) [30], N fixations (biological N2 fixation (BNF)), and N losses (i.e., NH3 volatilization (NH3V), N2O emission (N2OE), and N leaching (NL)).

2.2. Data Acquisition and Analysis

The raw data were obtained numerically from the text and tables or extracted from the figures in the original papers with the GetData Graph Digitizer 2.26. For the multiple sampling dates, the result of the biochar’s effects on the different sampling times and the uppermost soil layer was chosen [24,31]. SE values were unified into the SD value [18].
The effect of biochar addition on crop productivity and the parameter of the N dynamic was evaluated using the response ratio (RR) [24,29]:
RR = ln ( X t X c )
where Xt and Xc are the results of the biochar and control treatments, respectively. Moreover, standard deviation and the number of replicates were used as a measure of variance. The weight for each effect size was considered as its inverse variance [19]. The effect sizes of the above categorized groups were calculated using a categorical random effects model [31,32].
Mean effect sizes of each category and the 95% confidence intervals (CIs) generated by bootstrapping (999 iterations) were calculated using MetaWin 2.1 software [24,29]. The effect sizes (RR) were converted to percentage change (Pc) as follows:
  P c = 100 % exp RR − 1
Mean effect sizes were considered significantly different from zero if the 95% confidence intervals (CIs) did not overlap zero and considered significantly different from each other if their 95% CIs did not overlap [24,29]. The mean of all the effect sizes combined was calculated for the soil N dynamics and crop productivity under biochar addition in the global field conditions.
To further evaluate the related data validity and the robustness of this meta-analysis, a fail-safe number and funnel plot were used to elucidate the publication bias [18,32], which was compared with 5n + 1 (n is the number of cases). The results of crop productivity and soil N dynamic parameters from the datasets were without publication bias (Tables S1–S3 and Figure S1). The between group heterogeneities (QB) were calculated to examine the heterogeneity between groups across all of datasets for a given response variable (see Figure 3) [24,32,33]. A quantification of the main controlled factors about the soil N dynamics affecting crop productivity under biochar addition was elucidated using the aggregated boosted tree (ABT) model.

3. Results

3.1. Effects of Soil N Dynamic on Crop Productivity under Biochar Addition

With regard to the parameters of crop productivity, biochar addition notably increased the CAB and crop yield by 13.4% and 12.8%, respectively, but did not change the CBB (Figure 2a). Biochar addition also significantly enhanced plant N uptake by 22.6% (Figure 2a). With respect to the parameters of soil N pools, TN, MBN, IN, and NH4+-N in the soil N pools under biochar addition were notably increased by 13.3%, 18.7%, 7.22%, and 12.3%, respectively, while NO3−-N was not significantly varied (Figure 3). The range of 21–40 t ha−1 of biochar load produced the greatest increase in TN (21.1%), and TN was significantly inhibited when the biochar application rate was greater than 40 t ha−1 (Table S4). For the N losses, N2O emission and N leaching were greatly reduced by 15.9% and 17.3% with biochar addition, respectively (Figure 3). For the N fixations, biochar addition significantly enhanced BNF by 15.7% (Figure 3).
The RR of plant N uptake was positively correlated with the RR of crop productivity (p < 0.001; Figure 2b). The response ratio (RR) of crop productivity was negatively and positively correlated with the RRs of BNF (p < 0.05) and TN (p < 0.05), respectively. There was no correlation between crop productivity and the RRs of MBN, IN, NH4+-N, NO3−-N, N2O emission, NH3 volatilization, or N leaching under biochar addition (Figure 4 and Table 1).

3.2. Effects of Major Control Factors on Crop Productivity under Biochar Addition

The ABT analysis was carried out to compare the relative importance (major influence factors) of the soil N dynamic parameters with biochar addition on crop productivity in the field conditions (Figure 4). In total, 88.4% of the variances in the crop productivity was explained by the first five factors of TN, BNF, MBN, NO3−-N, and IN. In terms of the relative influence of the top four factors, soil total N was the most influential variable on crop productivity (50.1%) under biochar addition among the nine chosen variables, followed by BNF (15.1%), MBN (8.83%), and NO3−-N (7.67%), in soil N dynamic processes (Figure 5).

4. Discussion

Our meta-analysis revealed that biochar addition facilitated the enhancement of crop productivity, with a significant increase in the crop yield and CAB but not in the CBB (Figure 2a). These findings were consistent with the results of previous meta-analyses, which showed that the specific properties of biochar (e.g., high liming effect, available nutrients, and structure porosity) could ameliorate soil biochemistry and texture to improve crop productivity [9,10,19]. A recent meta-analysis found that biochar addition averagely increased crop productivity (yield) by 23.0% [22]. While the enhanced crop productivity by biochar addition in our study (by 12.8%) was significantly lower than the above result, it was similar to the results of other meta-analyses (about 10%) [9,19]. These results suggest that biochar addition can significantly increase crop productivity by at least by 10% in the global field conditions.
This meta-analysis also showed that soil N dynamic processes greatly changed with biochar addition in the global field conditions, including the increase of soil TN, IN, MBN contents, and plant N uptake, and the decrease of soil N2O emission (Figure 2 and Figure 3). The results were in accordance with those of previous meta-analyses [17,23]. The native nutrients (i.e., organic N and IN) of biochar addition to soil can directly increase soil TN and IN contents [18,34]. Meanwhile, the liming effect and structural characteristics of biochar significantly improved the soil physicochemical properties, and then promoted soil microbial abundance and activity to mineralize organic N and enhance the amount of the available N, IN, and MBN in the soil [16,35,36,37,38]. Our previous meta-analysis found that a biochar application rate of 21–40 t ha−1 produced the greatest increase in soil N-cycling microbial and enzyme activities, which can be beneficial to mineralize organic N and improve the amount of TN, soil available N, and MBN (Table S4) [24]. Moreover, biochar addition greatly accelerated the exudation of N coming from plant roots, and then increased soil TN, IN, and MBN [10,20,39,40]. These were beneficial to improve the N use efficiency of plants and plant N uptake. N2O can be adsorbed by biochar and/or may be reduced to N2 by the conversion of N2O to N2 (an increased abundance of N2O-reducing bacteria in certain cases) in the denitrification [41,42,43,44], thus reducing N2O emission under biochar addition (N2O emission saw the greatest decrease when the biochar load was 21–40 t ha−1) (Table S4); this further provides a soil N source for crop uptake and growth (Note S2).
Previous studies showed that the soil N dynamics generally have a strong influence on crop productivity [10,45]. Our results indicated that the increase of soil TN content with biochar addition was more beneficial to improve crop productivity compared with other indexes of N dynamics (Figure 4a and Table 1). High soil N content, which provides an adequate nitrogen source or translates into more available N to be extracted by plants, could significantly enhance plant N uptake and thus improve crop productivity under biochar addition. We also found that improving plant N uptake significant contributed to increasing field crop productivity under biochar addition (Figure 2b), which was consistent with the results of previous studies [16,17,46]. This indicates that biochar combined with N fertilizer application could highly reduce N leaching, improve N use efficiency and crop productivity, and reduce the emission of greenhouse gases [17,24]. Nevertheless, the contents of soil NH4+-N and NO3−-N, which act as the forms of available N that can be directly utilized by crops (plant N uptake), were not noticeably correlated with crop productivity (Table 1), which was possibly attributed to the different preferences for soil diverse N forms during the growth of disparate crops [10,46]. For instance, soil NO3−-N was reported as the “preferred” mineral N source for wheat growth [47,48]. Other studies showed that the high content of soil NO3−-N significantly improved maize growth and productivity but reduced rice productivity under biochar addition [13,49].
Moreover, our meta-analysis showed that soil IN (i.e., NO3−-N, NH4+-N) was not the major influence factor for crop productivity under biochar addition (Figure 5). This suggests that soil IN did not greatly or directly affect crop productivity under biochar addition. It is possible that the increased part of soil IN could be directly used by the crops (translating into the effect of plant N uptake), and thus did not significantly relate to the enhancement of crop productivity under biochar addition. However, previous studies found that the contents of soil total N and IN played important roles in the plant N uptake and crop productivity with biochar addition [17,18,50]. These implied that the major control factor of plant N uptake for increasing crop productivity could have other internal mechanisms (Figure 5), such as the mineralization of soil microbes for organic N in the soil TN [24]. For instance, some studies demonstrated that the mineralization of soil microbes to the organic N (MBN represents an important response index) greatly influenced and increased plant N uptake under biochar addition [16,24,51,52], especially when 21–40 t ha−1 of manure biochar was applied into the field soils, which could result in the greatest enhancement of plant N uptake [17,24]. Meanwhile, in our study, the MBN also greatly affected crop productivity under biochar addition (Figure 5). This implies that the MBN or N mineralization of microorganisms should indirectly influence crop productivity by the major function of plant N uptake, acting as a bridge to affect crop productivity under biochar addition [16,24,35,38].
According to the ABT analysis, we found the non-significant effect of N2O emission, NH3 volatilization, and N leaching on crop productivity under biochar addition (Figure 5 and Table 1). This could be mainly because NH3 volatilization and N leaching were obviously and gradually decreased over time (i.e., the long-term field experiments) by biochar addition [53,54,55], which could lead to the lower and continuous influence on crop growth. N2O emission was greatly reduced in a short time and then was beneficial to increasing crop productivity [17], but could be increased in the field over time with biochar addition and then may decrease crop productivity [24]. Thus, the effect of N2O emission to crop productivity could be non-significant under biochar addition. The above analyses further suggest that the regulation of soil total N content and plant N uptake could be the most effective factors to improve crop productivity under biochar addition in global field conditions.
Peculiarly, biological N2 fixation significantly influenced crop productivity by biochar addition (Figure 5) due to the liming effect, high porosity, and nutrients of biochar, improving soil pH, available nutrients, and physical properties, and thus increasing nodule growth and crop (i.e., legumes) productivity [17,56,57]. However, we found that biological N2 fixation was negatively correlated with crop productivity under biochar addition (Figure 4b and Table 1). The possible reason is that the significant improvement of soil N content (i.e., TN, NO3−-N, and IN; Figure 3) in this meta-analysis could directly inhibit the nutrient source of biological N2 fixation under biochar addition. For example, a previous study indicated that the reduction in free soil NH4+-N and NO3−-N under biochar addition enhanced the percentage of biological N2 fixation [58]. Moreover, some studies indicated that high soil N content led to the soil phosphorus limitation [59]. However, the phosphorus limitation further greatly reduced biological N2 fixation and then affected plant productivity in terrestrial ecosystems [60]. Some studies found that the high amount of soil available phosphorus greatly increased biological N2 fixation, and then improved the corresponding crop yield and productivity under biochar addition [58,61,62]. These results imply that the improvement of soil N content limited soil phosphorus availability for the response of biological N2 fixation, and then reduced the N2-fixing crop productivity under biochar addition. Therefore, biochar combined with phosphorus fertilizer application could greatly improve the biological N2 fixation of crops, which is beneficial to increase N use efficiency and N uptake and thus enhance crop productivity.

5. Conclusions

Based on this meta-analysis using field data sets across the global scale, biochar addition significantly changed N dynamic processes and enhanced crop productivity. The positive correlation between crop productivity plant N uptake and biochar addition suggested that biochar-promoted crop N absorption favors crop growth. Our results also indicate that soil TN content was positively correlated with crop productivity, but biological N2 fixation was negatively correlated with crop productivity. Combined with ABT analysis, we further found that the changes of soil total N content and biological N2 fixation in the N dynamic processes were the most important influence factors to enhance crop productivity under biochar addition in the global field conditions. These findings suggest that biochar addition could have contrasting effects on leguminous and non-leguminous crop productivity through increasing the soil N resource and N uptake. Nevertheless, it should be noted that the majority of field studies data used in the present study were acquired from China (with extremely limited data from other countries), which confines the data to a specific range of soils and climate conditions and thus influences the generality and representativeness of our conclusions. Therefore, it is necessary to conduct more extended field experiments to examine the different factors and long-term effects of the soil N dynamic on crop productivity (especially for different types of crops under different soil and climate conditions) under biochar addition at the global scale.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/agronomy12020247/s1, Figure S1: Funnel plots of the effect sizes of abundance of a) crop productivity, (b) crop belowground biomass (CBB), (c) crop aboveground biomass (CAB), and (d) crop yield (Yield) in the investigated datasets, Table S1: Values of fail-safe number of crop productivity (CP) (CP including crop aboveground biomass (CAB), crop belowground biomass (CBB), and yield) in the investigated datasets, Table S2: Fail-safe number values of soil total N (TN), microbial biomass N (MBN), NH4+-N, NO3--N, and inorganic N (IN) in the investigated datasets, Table S3: The values of fail-safe number of N2O emission (N2OE), NH3 volatilization (NH3V), soil N leaching (NL), biological N2 fixation (BNF), and plant N uptake (PNU) in the investigated datasets, Table S4: Values of the mean and 95% confidence interval (CI) of percentage changes (Pc: %) of soil total nitrogen (TN), biological N2 fixation (BNF) and N2O emission under different biochar load (t ha-1). n is sample size; Note S1: Data sources; Note S2: Results. References [13,14,26,28,30,46,49,51,58,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147] are cited in the supplementary materials.

Author Contributions

Conceptualization, L.Z. and M.Z.; data curation, L.Z. and Y.J.; formal analysis, J.L.; funding acquisition, L.Z. and Q.D.; investigation, Y.L. and B.Y.; methodology, L.Z. and Y.X.; project administration, Q.D.; supervision, Q.D.; visualization, L.Z.; writing—original draft, L.Z. and M.Z.; writing—review and editing, Q.D. and L.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study was jointly supported by grants from the Fundamental Research Funds for the National Natural Science Foundation of China (32101397, 31870461), the Guangdong Basic and Applied Basic Research Foundation (2021A1515011559), the Research Fund Program of the Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (2020B1212060022), and the “Hundred Talent Program” of the South China Botanical Garden at the Chinese Academy of Sciences (Y761031001).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article and its Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Global distribution of the data sampling sites for soil N dynamic parameters and crop productivity of global field experiments under biochar addition in this meta-analysis. The red points represent each data sampling site in the different countries.
Figure 1. Global distribution of the data sampling sites for soil N dynamic parameters and crop productivity of global field experiments under biochar addition in this meta-analysis. The red points represent each data sampling site in the different countries.
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Figure 2. Means and 95% confidence intervals of the percentage changes (%) of biochar effects on (a) plant (crop) N uptake and crop productivity including crop yield (Yield), crop aboveground biomass (CAB), and crop belowground biomass (CBB); the sample size of each variable is displayed over the corresponding bar. (b) Relationships between the response ratios of crop productivity vs. plant (crop) N uptake.
Figure 2. Means and 95% confidence intervals of the percentage changes (%) of biochar effects on (a) plant (crop) N uptake and crop productivity including crop yield (Yield), crop aboveground biomass (CAB), and crop belowground biomass (CBB); the sample size of each variable is displayed over the corresponding bar. (b) Relationships between the response ratios of crop productivity vs. plant (crop) N uptake.
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Figure 3. The percentage changes (%) of biochar effects on the parameters of N dynamics including soil total N (TN), microbial biomass N (MBN), inorganic N (IN), ammonium N (NH4+-N), nitrate N (NO3−-N), N2O emission (N2OE), N leaching (NL), NH3 volatilization (NH3V), and biological N2 fixation (BNF). Bars indicate 95% confidence intervals. Data at right side of each panel represent the number of observations.
Figure 3. The percentage changes (%) of biochar effects on the parameters of N dynamics including soil total N (TN), microbial biomass N (MBN), inorganic N (IN), ammonium N (NH4+-N), nitrate N (NO3−-N), N2O emission (N2OE), N leaching (NL), NH3 volatilization (NH3V), and biological N2 fixation (BNF). Bars indicate 95% confidence intervals. Data at right side of each panel represent the number of observations.
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Figure 4. Relationships between the response ratios of crop productivity vs. (a) soil total N and (b) biological N2 fixation under biochar.
Figure 4. Relationships between the response ratios of crop productivity vs. (a) soil total N and (b) biological N2 fixation under biochar.
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Figure 5. The relative influence (%) of the effects of soil N dynamics (i.e., microbial biomass N (MBN), inorganic N (IN), ammonium N (NH4+-N), nitrate N NO3−-N), NH3 volatilization (NH3V), N2O emission (N2OE), N leaching (NL), biological N2 fixation (BNF)) under biochar addition on crop productivity based on aggregated boosted tree (ABT) model analysis.
Figure 5. The relative influence (%) of the effects of soil N dynamics (i.e., microbial biomass N (MBN), inorganic N (IN), ammonium N (NH4+-N), nitrate N NO3−-N), NH3 volatilization (NH3V), N2O emission (N2OE), N leaching (NL), biological N2 fixation (BNF)) under biochar addition on crop productivity based on aggregated boosted tree (ABT) model analysis.
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Table
Table 1. Pearson correlation coefficients (r) between the response ratios (Equation (1)) of crop productivity and soil nitrogen (N) dynamic parameters. The “n” represents sample size. * represents the significance levels of p < 0.05.
Table 1. Pearson correlation coefficients (r) between the response ratios (Equation (1)) of crop productivity and soil nitrogen (N) dynamic parameters. The “n” represents sample size. * represents the significance levels of p < 0.05.
Indexrp Valuen
Soil total N0.1450.012 *303
MBN0.0040.98138
Inorganic N0.0090.905166
NO3−-N−0.0010.993192
NH4+-N0.0130.864170
N2O emission−0.1370.053200
NH3 volatilization0.0480.73552
N leaching−0.2400.23726
Biological N2 fixation−0.3370.048 *35
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Zhang, L.; Zhang, M.; Li, Y.; Li, J.; Jing, Y.; Xiang, Y.; Yao, B.; Deng, Q. Linkage of Crop Productivity to Soil Nitrogen Dynamics under Biochar Addition: A Meta-Analysis across Field Studies. Agronomy 2022, 12, 247. https://doi.org/10.3390/agronomy12020247

AMA Style

Zhang L, Zhang M, Li Y, Li J, Jing Y, Xiang Y, Yao B, Deng Q. Linkage of Crop Productivity to Soil Nitrogen Dynamics under Biochar Addition: A Meta-Analysis across Field Studies. Agronomy. 2022; 12(2):247. https://doi.org/10.3390/agronomy12020247

Chicago/Turabian Style

Zhang, Leiyi, Meixia Zhang, Yantao Li, Jianling Li, Yiming Jing, Yangzhou Xiang, Bin Yao, and Qi Deng. 2022. "Linkage of Crop Productivity to Soil Nitrogen Dynamics under Biochar Addition: A Meta-Analysis across Field Studies" Agronomy 12, no. 2: 247. https://doi.org/10.3390/agronomy12020247

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