How does N mineral fertilizer influence the crop residue N credit?

: In no-tillage systems, there is an accumulation of crop residues (CR) which plays an important role in available of soil-N. A study was set up to provide information regarding the CR N credit, and the influence of N mineral fertilizer. An incubation was run in a loam soil with addition of a similar rate of residue (10 Mg ha -1 ; sugarcane, soybean, and brachiaria) and N mineral fertilizer (urea; 120 kg N ha -1 ). After the stabilization of biological activity, soil and remaining residues were collected, and N monitored. Results showed that the N credit was positive with application of soybean, sugarcane, and brachiaria. There was an expressive performance of soybean N credit represented by a positive balance, and a reduction from 2.49 to 0.9 g kg -1 of N in residue, with direct increase of 90% of soil-N. There is no need of N fertilizer to potentialize the soybean N credit, but it is requested to potentialize N credit of brachiaria and sugarcane. The urea demonstrated be a good enhancer of brachiaria N credit, but it was not adequate to sugarcane residues. Based in our result, the accumulation and incorporation of CR can be considered as a N credit with positive contribution in soil-N.


Introduction
In no-tillage systems, there is an accumulation of crop residues which drives the decomposition kinetics and impacts the dynamic of nutrients in soil with the mineralization and/or immobilization of nutrients. The accumulation of crop residues also increases the soil quality, impacting positively the aggregation, water retention and cation exchange capacity in soil with a direct effect on crop production and the diversity of fauna and flora [1][2][3] . In Mediterranean climate, 4 showed an increasing of soil quality in a no-tillage system during 15 years, represented by a higher content of organic matter and the abundance and diversity of micro-arthropods. In tropical climate, also is commonly the increase of residues in soil in no-tillage systems, but with a rapid decomposition due to the adequate conditions of temperature and moisture to biological activities 2,5 . volatilization 13 .
The uses of legume cover crops have been presented as alternatives to enrich soil N by accumulation and decomposition of remaining residue. The term "N credit" has been associated to soil N derived from cultivation of soybean (Glycine max L.) in succession systems of soybean and corn (Zea mays L). The efficiency of N credit is variable due to the direct influence of biological activity in N mineralization. Studies of 2,14,15 showed that the rate of residue mineralization in soil depends of temperature, soil moisture, and management (i.e. rate, quality/source, and incorporation) of crop residues. 16 showed that in a sandy soil, soybean remaining residue provided little N to subsequent crops due to N losses by leaching; but there was an increase of corn yield and N uptake in subsequent crop in a silt loam soil. 17 noticed that the use of sunn hemp (Crotalaria spectabilis) as cover crop promoted sugarcane (Saccharum officinarum) yield in subsequent years, but there was no effect in content of soil N. Possible, the plays role of N credit of legumes (i.e. soybean; sunnhemp; bean, Phaseolus vulgaris; and lentil, Lens culinaris), not only soybean, also is associated to the quality/quantify of N mineral fertilizer in subsequent years. 4 showed that the application of Vicia villosa (legume), known as the hairy vetch in Europe/Western Asia, can be considered as a N organic source and its effect is enhanced when associated with a moderate N rate.
Even with positive benefices of crop residues in N dynamic, the application of mineral fertilizer is commonly performed in tropical condition after legumes planting without counting the N credit.
Possibly, studies that demonstrate the legume N credit and influence in N soil and crop subsequent yield can contribute to decrease the use of mineral N and improve N credit supplied by legumes in crop sequences. It seems reasonable the reducing of conventional N fertilizers by organic N source derived by legume residues. If success the association of N sources (organic and mineral), isolated or associated, will achieve the principles of a circular economy with an adequate balance of society, economy, and environment 18,19 .
This study was set up to provide information regarding the potential of crops residues N credit, as a complementary N source associated with N mineral fertilizer. We hypothesized that the addition of crop residues (soybean, brachiaria and sugarcane) can be an alternative to increase the N content in soil. The goal here was to monitor the inputs of N in soil by the application of crop residues and N mineral fertilizers, and theirs impact in content of soil N and remaining residues N after the stabilization of biological activity in soil.

Experimental characterization
A study of incubation was run with additions of residues (soybean, brachiaria and sugarcane), and N mineral fertilizer in soil, isolated or associated, using three replications in a completely randomized block design. Soil without inputs of residues or N mineral fertilizer was monitored as a control.

Soil and residues incubation
A soil sample of 700 g (size < 2 mm) was added in a column (volume: 1298.8 cm 3 ; height: 13 cm; diameter: 10.5 cm) and moistened at 60 % of field capacity. An equivalent average rate of 10 Mg ha -1 (17 g of residue pot -1 ) of residue of soybean, sugarcane and brachiaria were incorporated and homogenized in soil surface (at the first 5 cm) to promote the contact of soil and residue. We used this similar residue rate to understand the effect of residue quality in N content in soil. In field, the residue accumulation average of sugarcane, soybean, and brachiaria, respectively, ranging from 15 to 20 25 , 2 to 4 26 , and 5 to 12 Mg ha -1 27 . A rate of 120 kg ha -1 of N mineral fertilizer (urea: 45% N) was incorporated and homogenized also in soil surface (at the first 5 cm). The N rate was based in an average of current N recommendation for crop productions in tropical conditions.
Column was fixed on a tyrofoam base properly insolated to prevent soil water loss and keeping at 60 % of field capacity, which was constantly weighted with water addition when requested. This study was developed in incubation due to the controlled experimental conditions as demonstrated by 15,28

Measurements and statistical analysis
After the stabilization of biological activity with 73 days of incubation, soil and remaining residues were collected and sieved (2 mm mesh). Soil and remaining residues were physically separated and content of total N, in each part, monitored using the Kjeldahl method 20 . The balance of N in soil and residues were calculated according to the difference of N contents in 73-day and initial N content (without N addition in soil).
The variability of the variables was studied using descriptive statistics (average, standard deviation, minimum, maximum and median values). Assumptions of residues normality and variances homogeneity were tested using the Shapiro-Wilk test, and the Bartlett test, respectively (P ≤ 0.05). Data were submitted to the t test to compare two averages (average of treatments and control; P ≤ 0.05), and the LSD test to compare more than two averages (average of residues, N, residues and N; P ≤ 0.05).
A graphical algorithmic representation was adapted to demonstrate the inputs (crop residues and N mineral fertilizer) and outputs (N soil, N losses, and N in residue remaining) in the incubation system. The positive outputs were correlated using Pearson correlation (P ≤ 0.05). The Statistical Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 19 July 2020 doi:10.20944/preprints202007.0429.v1 analysis was performed using R Statistical Software (version 4.0.0; R Foundation for Statistical Computing).

Total soil N
With the general average, the application of crop residues increased 85% of N in soil followed by an increasing of 60% with the addition of N mineral fertilizer. The associated applications of both (crop residues and N mineral fertilizer) promoted a higher N inputs in soil with an increasing of 90% (P<0.05), and, therefore, indicating an accumulated and positive effect of both N sources in soil N content ( Figure 1A). Focusing in each crop residue, the highest soil N accumulation was found with application of soybean residue with a general average of 0.86 g kg -1 , followed by residue of brachiaria and sugarcane with respective general averages of 0.67 and 0.57 g kg -1 ( Table 1). The associations of N mineral fertilizer and residues of soybean presented the highest soil N inputs with an average of 1.3 g kg -1 , without difference to application of soybean residue isolated (average: 1.1 g kg -1 ), Figure 1B. Also, there was not a significant difference in application of sugarcane residue with a general average of 0.8 g kg -1 , and a small difference of 0.2 g kg -1 between the average of residue and residue + N ( Figure   1C). These results showed an N input efficiency from soybean residue, and a possible N loss in systems with application of N mineral fertilizer mainly associated with sugarcane residue. On the other hand, there was a significant effect of brachiaria residues and N mineral fertilizer which represented an increase of 42 % compared to application of residue isolated (P<0.05), Figure 1D.   The balance of soil N revealed that the associations of crop residue and N mineral fertilizer increased the N content in soil with an average ranging from 84 to 90%, considered 5% higher than residue application isolated ( Table 2). The highest positive balance of soil N was obtained with application of soybean residue with a sequence: soybean>sugarcane=brachiaria (residue), and soybean>brachiaria>sugarcane (residue + N mineral fertilizer). These results demonstrated that the soybean residue is a great N credit to increase soil N, and there was need of association with N mineral fertilizer represented by a positive balance.

Remaining residue total N
With general average, there was a decrease of 62% and 50% of N in remaining residue after the incubation, respectively with application of residue and residue + N. This result indicated that the association of residue + N promoted a higher N accumulation in residue (up to 0.2 g kg -1 ), if compared with incubation of residue alone (Figure 2A).
Focusing in each crop residue, the highest N accumulation in remaining residue was found by soybean with a general average of 0.89 g kg -1 , following brachiaria and sugarcane with respective average of 0.73; and 0.20 g kg -1 (Table 1). In all residues, there was a reduction of N in residue compared initial N after the incubation-time with negative balance ranging from -46 to -68 % ( Table   2).
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 19 July 2020 doi:10.20944/preprints202007.0429.v1 Table 2. Balance of total N in soil and remaining residue after the incubation of crop residues (R; soybean, sugarcane and brachiaria) and nitrogen mineral fertilizer (N). As expected, the N reduction in remaining residue was more expressive in incubation of soybean with a decrease of -61% (residue + N) and -68 % (residue), respectively representing a reduction from 2.49 g kg -1 of N in initial residue to 1.0 and 0.8 g kg -1 of N after the incubation time (Table 2; and Figure 2B). There was no difference of N in remaining residue with application of soybean and sugarcane residue, isolated or associated with N, represented by a general average of 0.9 and 0.6 g kg -1 of N after the incubation time, respectively ( Figures 2B and 2C). In contrast, the incubation of brachiaria, and brachiaria + N decreased -66 and -46% of N in remaining residue with an average from 1.7 g kg -1 of N in initial residue to 0.6 to 0.9 g kg -1 , respectively ( Figure 2 D). Therefore, these results indicated that there was N mineralization in soil, and a low N immobilization when associated to N mineral fertilizer. We also expected to find this result with application of sugarcane residue, but there was no a clear evidence of N immobilization with a general average ranging from 0.6 to 0.7 g kg -1 of N in remaining residue ( Figure 2).

Soybean
With the general mean of treatments, we observed that there was a positive correlation between the soil N and remaining residue N with a r of 0.99 with incubation of all residues (P ≤ 0.05). The inputs in systems of result from reduction of N in remaining residue in -68% and -56%, respectively when applied residue isolated or associated with N mineral fertilizer. Increasing of soil N with mineralization of soybean residues was presented by 14,15,42 . 30 showed that compost application of beans and soybean residues and manure increased 44 and 27% of total organic nitrogen in soil layer 0.0-0.1 and 0.1-0.2 m, respectively compared to soil without N application. In our study, the highest decrease of N in remaining residue corroborated the N addition in soil by mineralization of soybean with a correlation of 99% (Pearson correlation). The soybean residue presented a low C:N rate and high N available (2.49 g kg −1 ) to activity of microorganism in soil and N mineralization. Study of 43 with application of residues (rice straw, rice root, cowdung, and poultry manure) noticed that the high amount of N and low C:N ratio contributed to rapid microbial decomposition and increase of N in soil. Therefore, our results and results from literature 14,15,42 , indicated that the soybean residue can plays as a N source due to high N credit in soil. The application of N did not potentialize N credit of soybean residues with a small difference of 0.2 g kg -1 between residue and residue + N. In field, Blackmer (1996) showed  There was no difference between the application of sugarcane residues isolated or associated with N. The low N contribution with applications of sugarcane residue also was demonstrated by 31 .
In tropical conditions, 46 using 15 N showed a total of 13% of N derived from sugarcane residues incorporated into the in short-term. Similar results was presented by 31 in a wet tropical of Australian with a modest contribution of N from sugarcane residue. These results are explained by a high C:N ration and low N content (1.44 g kg −1 ) in residue that possibly promoted a immobilization of N in residue. 47,48 demonstrated that the high C:N in residues is associated with content of cellulose, hemicellulose, and lignin. The residues with high C concentrations, as sugarcane, the microorganisms request several cycles and an extra time to decompose it 43 . 46,49 described that the immobilization of N in sugarcane residue plays as a reserve of N turning a slow release source in long-term. Interest, we expected that application of N would promote the increase of mineralization and content of N in soil. However, in our study we did not notice this result with a small difference of 0.2 g kg -1 between residue and residue + N. Possibly this result can be associated with the low rate of residues tested, which was 50% lower than in the field (green harvest), and application of N as a source of urea; which is characterized by high losses of N through ammonia volatilization, mainly when surface-applied to soils 50-53 . 54 showed that N lost by application of N as urea can achieve 35% of N applied when associated with sugarcane residue. Another perspective can be the physical separation of residue and soil in the collecting after the incubation, probably for next studies will be request the physical separation of residue and soil, and a washing of residue to avoid residue contamination with soil. residue is associated with quantify of residues added in soil 15 . The results of 29 were explained by high residue rates. 2 also showed that N addition is a great strategy to increase the N stocks in soil and avoid the soil depletion.

Conclusions
The N credit of incorporated crop residues was positive with application of soybean, sugarcane and brachiaria residue. There is an expressive performance of soybean residues in N credit represented by a positive balance with a reduction from 2.49 g kg -1 of N in initial residue to an average of 0.9 g kg -1 of N in remaining residue after the incubation time, representing an increase of 90% of soil N. There is no need of N mineral fertilizer to potentialize the N credit with incorporation of soybean residues, but it requested with accumulation and incorporation of brachiaria and sugarcane residues in soil. The urea demonstrated be a great enhancer of N credit in brachiaria residue, but it is not adequate to sugarcane residues due N losses by volatilization that is well-demonstrated in literature [52][53][54] . Based in our result, the accumulation and incorporation of crop residues can be considered as an N organic source with positive contribution in soil N due to mineralization of organic matter.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 19 July 2020 doi:10.20944/preprints202007.0429.v1 Author Contributions: All authors conceived and ran the experiments, and contributed to the interpretation of the results, provided critical feedback, and helped shape the research, analysis and manuscript.