1. Introduction
Soils are the largest reservoir of carbon (C), and C sequestration in soils can help mitigate climate change [
1]. Meanwhile, little is known about the magnitude of change in C stocks due to long-term soil conservation strategies in dry areas. According to Sapkota et al. [
2], limited work has been done on quantifying the long-term tillage and straw management effects on soil parameters, especially C on marginal soils in dry areas, where there are challenges in generating adequate amounts of biomass. The soil organic C has various pools that differ in decomposability [
3,
4]. The recalcitrant or passive pool is organic C that is resistant to further biodegradation but is important in C sequestration [
3,
4]. The active pool is easily decomposable and is an important source of plant nutrients, especially phosphorus (P) [
4]. In any given crop production system, the concentration and storage of C can be affected by the tillage intensity and frequency, crop residue management, fertilizer input, and interactions of the aforementioned factors [
5,
6]. No-tillage is known to reduce losses of sequestered C from tropical soils as it slows down the decomposition rate of soil organic matter (SOM) [
7,
8,
9]. On the other hand, crop residues accumulate on the surface under no-tillage maintaining higher soil carbon stocks [
10]. However, this may increase losses of crop residues to wind erosion on open plains, and consequently, increase C losses when compared to stubble mulched and ploughed systems.
The Eastern Free State of South Africa is a semi-arid temperate area, with dry winter months, summer rainfall, and a high wind erosion hazard [
11,
12,
13]. In this region, the production of dryland wheat (
Triticum aestivum L.) during the dry winter months is made possible through the appropriate timing of planting on plinthic soils that have high water tables and high moisture-storage efficiency. The average yields of this wheat that are produced under severely water-limited conditions are low at 2.58 Mg ha
−1 [
14]. To reduce the production costs of the dryland wheat, farmers in the region use a low input production system, which entails reduced fertilizer and pesticide inputs and at times straw burning for plant disease control. Dryland wheat production in the Free State was estimated at approximately 450,000 ha in 2005, contributing to nearly 50% of South Africa’s domestic wheat requirements [
15]. However, the production area has declined to under 100,000 ha at present [
16]. Many farmers lost interest in dryland wheat production due to yield and profitability challenges [
14].
Apart from frequent droughts, gradual nutrient depletion is a major threat to sustained dryland wheat production on plinthic soils that are low in soil organic carbon (SOC) and cation exchange capacity (CEC), and are highly susceptible to erosion. Soil acidity (including subsoil) also develops when plinthic soils are cultivated, resulting in aluminium (Al) toxicity and poor root growth. According to Fey [
17], the surface horizons of plinthic soils that have low SOC and high iron oxides degrade easily under excessive tillage. Excessive tillage increases the susceptibility of soils to wind erosion, resulting in increased loss of particulate organic matter and other nutrient-rich sediments of the topsoil [
12,
13]. If nutrient losses from the soil are not abated, fertilizer costs could increase, thus reducing profitability. Suggested strategies for arresting soil degradation in wheat production systems include conservation agriculture (CA) practices, such as crop straw retention and no-tillage (NT) [
18,
19,
20]. These practices are envisaged to improve the SOC, thus possibly countering the long-term nutrient depletion effects of low-fertilizer input farming as shown elsewhere [
9,
10,
21,
22]. However, for dryland wheat farmers who have adopted CA in the Free State, it can be challenging to retain crop residues (wheat straw) as surface mulch because the strong winds blow everything away. Thus, light tillage to incorporate crop residues into the surface soil (stubble mulching) immediately after harvest would be important for crop residue protection against wind erosion.
Phosphorus is the most important nutrient in optimizing wheat growth, grain yield, and quality after N, yet most soils under wheat production in the tropics are P fixing in nature, and deficient in P [
23]. The retention of crop residues increases SOC, and potentially decreases the P adsorption capacity of soils through complexation of soluble Al, among other mechanisms [
23,
24]. Organic P sources favor the build-up of labile P pools at the expense of recalcitrant P when compared to inorganic P sources [
25]. However, the magnitude of such effects is likely to be variable depending on the interactions of various factors such as residue type and quantity, tillage intensity, soil and climatic conditions, and the duration of the crop production system. Understanding the effects of CA options on SOC stocks and fractions of P in dryland wheat production systems thus would require long-term, multi-factorial trials.
The only long-term experiment for evaluating the effects of dryland wheat crop management strategies on the soil in semi-arid regions was established in 1979 at the Small Grain Institute (SGI) in South Africa. This trial was originally established to investigate the effects of tillage, crop residue (hereafter referred to as straw), fertilizer application, and weed management on the yield of continuous wheat. Measurements were done on the total SOC changes in the aforementioned trial after 10 [
26], 20 [
27], 30 [
28], and 37 [
29,
30,
31] years of trial inception. Based on a distillation of research findings that were produced since the trial’s inception, it can be noted that in 1989, after 10 years of experimentation, Wiltshire and du Preez [
32] showed that the soil N and C did not vary with tillage or straw management treatments although SOC in the trial was lower compared to natural pasture near the trial. In 1999, after 20 years, Du Preez et al. [
26] investigated the same treatment effects and reported that the soil quality parameters had declined across all the treatments. After approximately 30 years of the trial, Loke et al. [
28] reported that not burning wheat straw resulted in lower extractable P but higher total N, when compared to burning, and that no-tillage (NT) accumulated more SOC in the topsoil (0–50 mm) compared to other tillage practices while both NT and stubble mulching (SM) enhanced the total N, soil pH, and P availability [
33], some of these results were in agreement with Motema et al. [
30]. All these studies were done with treatments where fertilizer N was applied at 40 kg ha
−1, with none done with treatments that were fertilized at 60 kg N ha
−1, which could increase biomass input. However, no investigations have yet been done on the sustainability of the various production systems in terms of C stocks, including charcoal-associated C, which could be associated with straw burning.
Soil C and P fractions are pivotal in explaining soil productivity and the quality of SOM being generated by various cropping systems, including in the long-term wheat trial in South Africa. It was, therefore, important to determine the long-term interaction effects of tillage and straw management on SOC stocks, fractions of SOC and P as the conclusion of this long-term trial after 40 years in 2019 approached. The specific objective of this study was to determine the effects of tillage and straw management on SOC stocks and P fractions. It was hypothesized that long-term reduced tillage would significantly increase the SOC fractions, C stocks, and P fractions, with the magnitude of benefit being dependent on the straw management strategy.
4. Discussion
Conventional farming practices, based on extensive tillage of the soil are reported to be the major cause of land degradation and SOC loss in the Eastern Highveld of South Africa [
46]. In the current study, we hypothesised that 40 years of reduced tillage practices (NT and SM) would reduce the loss of C and soil fertility that is associated with conventional tillage practices, with the magnitude of benefit being dependent on the straw management strategy.
The lack of effects of tillage and straw management on soil organic C could be a result of low biomass input from the dryland wheat under semi-arid conditions. The higher C concentration in the 200–400 mm soil layer of the burned plots than in the non-burned plots (
Table 1), suggests that straw burning increases C sequestration at deeper soil layers in dryland wheat production systems under semi-arid conditions. This observation could be a result of higher root biomass production under higher pH and labile P (Bray 1 P and NaHCO
3 Pi), which are readily available. The reduction of POC in the top 200 mm by CT especially when straw was burned, was explained by excessive soil disturbance, which resulted in the degradation of the labile C fraction in the topsoil. The higher macro POC under NT followed by SM suggested this labile form of soil C accumulated at the surface with less soil disturbance, while CT allowed the burial of material into the soil [
47]. The accumulation of POC under NT is in agreement with the findings by dos Reis Ferreira et al. [
48]. The increase in this labile form of C under NT and SM, suggests that these tillage treatments encouraged nutrient cycling including P, and these findings are supported by the significant correlations between the labile C and P fractions. The lack of differences in the charcoal C as a result of tillage or straw burning agrees with Rumpel [
49], who reported no significant effects of stubble burning in the aromatic and recalcitrant black carbon after 30 years of experimentation in France. The lack of differences was attributed to the low intensity of fire that is used for straw burning [
50], which could have resulted in little production of recalcitrant black carbon on the burned treatments. The accumulation of soil organic C and its labile fractions in parts of the soil profile may have affected the total C stocks (0–1000 mm depth).
An unexpected, but significant result from this study was that after nearly 40 years of wheat mono-cropping, the total soil C stocks (0–1000 mm) were higher under CT and SM with no burning as well as NT and SM with straw burning, than NT with no burning or CT with straw burning. A possible explanation for the lower C stocks overall on the NT with no burning treatment is that this treatment had significant losses of C in the form of straw that was blown away by the wind [
12]. Strong winds are a major production challenge in the wheat production region around Bethlehem, South Africa [
12,
13]. The NT without straw burning and CT with burned straw accumulated very little C stocks in t ha
−1 yr
−1, as outlined above due to straw being blown away by the wind under NT as well as a higher aeration rate under CT. As a result, the overall C stock under these management practices was lower compared to NT with straw burning, CT without straw burning, as well as SM. Stubble mulching, which refers to the slight incorporation of the straw into the soil immediately after harvesting grain was probably beneficial for protecting the straw against wind erosion [
8,
10], hence the higher C stocks on SM treatments with no burning. This also applies to CT without burning, where more biomass that was incorporated in the soil is protected from wind erosion loss. Burning of straw reduced biomass that was incorporated under CT, and with increased aeration of the soil due to the tillage effect, this could have lowered the total soil C stocks on treatments where CT was combined with burning. The higher C stock under NT with burned straw could be explained by the higher SOC in the subsoil, which should have been facilitated by greater available P under less acidic conditions. This reasoning also applies to SM with burned straw.
The aerobic combustion of straw produces alkaline ash, which may be the reason for increased mean pH in the burned systems at both 0–50 and 50–200 mm soil layers. Carbonates that are released after burning increase the soil pH [
51,
52]. During this process, organic P is converted to inorganic P, making P more available on the burned treatments [
53]. Higher P in the surface soil under NT and SM could be explained by the surface accumulation of OM under NT and SM practices. Since the burning of straw increases soil pH compared to no-burning, burning would also increase the availability of some nutrients, particularly P. The higher soil pH and available P where straw was burned in the NT treatment could also have provided more favorable conditions for root growth. The organic matter from roots and their exudates could have significantly contributed to mineral-associated C, which is supported by the strong correlation between the soil organic carbon and the mineral-associated carbon.
The higher Bray 1 P under NT followed by SM suggested that available P accumulated at the surface with less soil disturbance, while CT allowed the burial of material into the soil [
47]. Contrary to other parameters, NaHCO
3 P (labile) was higher under the burned CT treatments while burning wheat straw increased Al-associated NaOH I Pi, suggesting that straw burning increased the pH and made P more available by reducing P that was bound to Al. This view was supported by higher soil pH where straw was burned. The higher NaHCO
3 Pi in the burned CT treatment was in agreement with the findings by Romanya et al. [
53], who also reported higher labile P on the burned treatment.
When compared with the grassland soil nearby, all the treatments at least doubled the concentration of available P (
Table 3), under the annual input of 12.5 kg P ha
−1 year
−1 and 60 kg N ha
−1. However, only the NT practices had adequate available P (40 mg kg
−1) in the surface soil (
Table 3). This is supported by the NaOH II Pi (physically protected P), which was higher under NT and SM. The findings of the current study on NaHCO
3 Pi were comparable to Ncoyi et al. [
31], who reported 5.07 mg kg
−1 higher under SM and NT practices than CT, in the plots that were fertilized with 40 kg N ha
−1. This suggests that NT is a more sustainable approach for managing P depletion and reducing external P fertilizer requirements. It should be noted that where the soil was less disturbed, nutrient removal in grain was limited as shown by relatively lower yields that were obtained under NT compared to CT [
29]. Low nutrient removal rates probably explain why the concentration of available P in the soil remained adequate for dryland wheat after 40 years of continuous cropping, under NT [
54]. The lack of tillage effects on HCl Pi in the current study was contrary to those of Ncoyi et al. [
31], who reported a 2.16 mg kg
−1 higher concentration under SM and NT practices than CT, in the plots that were fertilized with 40 kg N ha
−1. The results of P fractions in the current study were generally higher than those that were reported by Ncoyi et al. [
31] due to the higher biomass production at 60 kg N ha
−1 than at 40 kg N ha
−1.