1. Introduction
Population growth is increasing the demand for agricultural products [
1,
2], which can increase land degradation due to intensive agricultural use, often of marginal areas. Land degradation impacts many soil physical, chemical, and biological properties, including organic carbon [
3,
4], hydraulic conductivity [
5,
6], bulk density, runoff and erosion [
6,
7], nutrient availability, microbial biomass, and enzyme activities [
8,
9]. Land degradation is a worldwide problem, particularly in sub-humid, semiarid, and arid regions [
1,
10]. In these regions, water shortage, due to rainfall scarcity and irregular temporal and spatial distribution, exacerbates the difficulty in maintaining and/or increasing agricultural yield. In the sub-humid and semiarid regions of Brazil, besides water problems, there are large areas with low fertility and low water retention capacity soils [
11,
12]. Cattle manure is commonly added to increase total organic carbon content (TOC) and nutrient availability, to reduce acidity, and to raise the water retention capacity. However, cattle manure availability is insufficient to fertilize all cropping areas and it usually has low mineral nutrient concentrations, due to the system of extensive cattle raising [
13].
The intensification of poultry production is providing another manure source [
14,
15], but this manure is not traditionally used in the region [
13]. As other manures, the concentration of nutrients is relatively low, implying high costs of transportation and distribution, and the added organic matter is usually mineralized within only a few cropping seasons. Therefore, applications have to be frequently repeated to maintain soil productivity [
16].
An alternative or complementary organic amendment is biochar (BC) [
1,
16,
17,
18,
19,
20]. BC is the result of the pyrolysis process of different biomass types, and it is rich in carbon [
21], although lower in nutrients than manures [
22,
23]. Positive effects of BC application on soil physical, chemical, and biological properties and crop yield have been reported in many studies [
18,
24,
25,
26,
27], although other studies show none or unfavorable consequences on soil properties and crop yield [
28,
29,
30,
31]. The amelioration of soil properties, such as decreased soil acidity and increased of nutrient concentrations and soil water retention, can be responsible for increased crop yields in soils amended with biochar [
18,
20,
24,
25,
26].
The scarcity of scientific reports in the study area that combined BC and poultry manure (PM) and simultaneously evaluated soil physical, chemical, and biological properties plus crop yields led to the establishment of the present work. To our knowledge, this is the first work to combine all these evaluation in a field experiment of a common bean crop under rainfed conditions in a tropical sub-humid climate. We hypothesized that the combination of BC and PM improves bean yield and soil properties in relation to the amendment with the isolated organic amendments.
3. Results
The soil amendments had no significant effects on BD, TP, FC, PWP, PAW, TOC, and MBC and also no effect on ET (
Table 3,
Table 4,
Table 5,
Table 6). The ranges of values were typical of this soil class in the region, with special notice to the low TOC concentrations (3.6 to 4.4 g kg
−1). The soil amendments had also no effect on soil pH and P concentration when compared to the absolute control, with exception of the treatments BC40 + PM and BC10 + PM, which significantly increased the pH and P concentration, respectively (
Table 4). The soil amendments had no significant effects on K concentration and on CEC (
Table 4). The treatments BC40, PM, and BC10 + PM had significant effects on P. Aci, and the treatments PM, BC10 + PM, and BC40 + PM on Ure enzymatic activities (
Table 5), when compared to the absolute control. The treatments BC10 + PM and BC40 + PM had significant effects on WUE and especially on the bean yield, which was more than three times higher with the treatments BC10 + PM and BC40 + PM (
Table 6), when compared to the absolute control.
Soil characteristic including pH, P, WUE, P. acid, and Ure were positively correlated with bean yield (
Table 7). These correlated characteristics were those that were improved by the soil amendments.
4. Discussion
The fact that the application of BC and PM, isolated or combined, did not enhance the soil physical properties, including BD, TP, FC, PWP, and PAW, could be partly attributed to the texture of the soil in the experimental area. The effects of these organic amendments are greater in coarse-textured soils [
27,
47] and the soil in the experimental area is classified as a sandy clay loam (
Table 1). Adekiya et al. [
22] found positive effects of BC and PM on physical properties of a Luvisol with 76% sand and Agbede et al. [
23] found that BC and PM treatments significantly reduced BD and increased TP and soil water content, also in a Luvisol, but with 92% of sand. The effects of the amendments are also influenced by their particle size and the 2 mm used in the current study may have been too coarse. Głąb et al. [
48] reported positive effects on soil physical attributes using particle size smaller than 0.5 mm. The short period (three months) between application of the amendments and soil sample may have also precluded more extensive changes in these soil properties [
49,
50]. Most studies describing positive soil structure evolution with BC amendment claim better soil aggregation due to enhanced microbial activity, the presence of mycorrhizal hyphae [
50,
51]. Finally, the applied amendments correspond to relatively small proportions of the total soil volume and mass of the sampled layer and, consequently, their influence on physical properties tends to be limited.
BD and TP are associated with soil texture and also with soil structure, i.e., aggregation and stability, which are related with TOC [
52]. Yang and Lu [
53] found that the soil aggregation and stability could be partly related to the increased TOC in the BC-amended soils. However, in our study, the TOC was not significantly influenced by the application of the soil amendments (
Table 4); thus, this can also explain the absence of significant effects of the amendments on BD and TP in this short-term field experiment.
The soil pH and P concentration, as well as enzyme activities were positively affected by the application of the soil amendments (
Table 4 and
Table 5), contrasting to the absence of effects on the physical properties. These effects are particularly important in acidic soils with low initial soil fertility status. Du et al. [
54] reported that PM application increases soil pH and Adekiya et al. [
22] and Agbede et al. [
23] argued that manure increases soil pH due to ion exchange reactions which occur when the terminal OH
− of Al
3+ or Fe
2+ hydroxyl oxides are replaced by organic anions, such as malate, citrate, and tartrate, originated from the decomposition of the manure. BC also increases soil pH due to its high pH (9.8), caused by the alkaline ash in its composition [
18], which includes Ca and Mg oxides and K oxides, hydroxides, and carbonates [
55].
The decrease in soil acidity may have promoted an increase in P availability (
Table 4). Glaser and Lehr [
56] found that the addition of BC significantly increased P availability, up to a factor of 4.6, independently of the woodstock used for BC production. The BC liming and fertilization effects, especially on nutrient-poor and acidic soils, results in higher crop yields [
57], as obtained in our study. Du et al. [
54] performed a meta-analysis to determine the effect of several manure types (e.g., pig, cattle, sheep, cow, chicken, livestock and farmyard) on soil properties and found that manure application also increases available P and K, promoting higher yields, more pronounced under warm and humid climates. The high correlation of the soil chemical and biological properties with yield indicates that the improvement in these properties may be partially responsible for the yield increase. The increase in pH and in nutrient availability may have also been contributed to the alteration in the Ure and P. acid activities, which are crucial for soil health due their role in organic matter decomposition, being considered the most sensitive indicator of soil changes [
58].
The most important effect certainly was the increase in bean yield (
Table 5). The average bean yield in this study region is low, around 900 kg ha
−1 [
59], and it is attributed to the sandy soils where the culture prevails, which have low fertility and low water retention capacity [
11]. The combined application of PM and BC increased the yield by 355%. In conjunction with the low income of farmers, the low availability of cattle manure [
13], and the good availability of PM and of agricultural wastes for the production of BC [
18,
60], the application of this combination can be a viable alternative to increase bean yield in the region.
The production costs of one hectare of bean, amended with 5 t ha
−1 of PM, in the study region, are approximately
$460 [
59], and considering that the price of bean is
$1.25 kg
−1, the application of PM would have a return of approximately
$1800 ha
−1. The BC cost is variable, but considering the value of
$200 ton
−1 [
61], the treatment BC10 + PM would have a return of approximately
$680 ha
−1, i.e., lower than that of PM alone. However, as BC is a source of recalcitrant carbon with a long mean residence time (several decades to several centuries) [
21], it can promote positive effects on soil properties for many years.