4.1. Effects of Harvest Residue Strategies on Soil C and Available Nutrient Stocks
The effect of forest residue removal on soil nutrients stocks is inconsistent between different studies in the literature. In some cases, no losses in soil C or nutrient stocks have been reported, even with increases in nutrients exported during harvest [4
]. This observation has been attributed to several mechanisms: high buffer capacity of the soil, slow decomposition of forest residues, a long harvesting return interval (more than seven years), and fast growth and litter deposition from new Eucalyptus plantations. However, reduction in soil C and nutrient stocks has been observed in wet tropical sites with sandy soils, high productivity forests, and successive harvest treatments [16
]. Our study agrees with this latter set of studies and shows substantial changes in soil C and nutrients following removal of harvest residues.
In this study, the highest stocks of oxidizable C in the soil were observed in the 0–20 cm soil layer of the ReM + F treatment, followed by ACR + F and ReR + F treatments (p
> 0.001). Similar results were observed by Moreno-Fernandez et al. [34
] in Mediterranean mountain Scots pine forests, where the highest stocks of C were in the 0–20 cm layer of the soil under moderate intensity management.
Stocks of soil C decreased as soil depth increased, with the lowest stocks being observed in the ACR − P treatment (p
> 0.006) at 150–200 cm depth. In a previous study developed in the same experimental site, Rocha [35
] found that the removal of forest harvest residues from the soil reduces the oxidizable organic C from the surface layer of the soil by 50%, and 75% of this reduction happens in its labile fractions. Vanguelova et al. [36
], on the other hand, concluded that there was no evidence that whole-tree harvest decreased soil organic C in a 28-year old second rotation stand of Sitka spruce in the UK. Slash removal has larger effects on soil C and nutrients when rotations are short, slash is removed repeatedly, clay content is low, temperatures are high, the site is relatively wet, and forest productivity is high [4
]. It is well known that retaining harvest residues on the soil is extremely important to maintain high productivity of tropical forests [14
]. However, more research is needed in order to reach a plausible conclusion about the subject due to the divergence in the data available, as observed by Nambiar and Harwood [14
The forest residue removal for two rotations resulted in the cumulative loss of 23.5 t·ha−1
of C from 0–200 cm soil depth (ReM + F and ReR + F treatments). This reduction was more concentrated in the top 40 cm (Figure 2
). This effect is a result of the additional removal of 80 t·ha−1
of biomass (slash and litter; 51 and 29 t·ha−1
in R1 and R2, respectively) in the ReR + F treatment. Furthermore, the reduced initial growth in the ReR + F treatment during both rotations resulted in less litter deposition [27
]. In the second rotation, a 40% reduction in fine roots was observed in the 0–30 cm layer of the ReR + F treatment [37
], which also contributed to low soil C content.
Forest residue removal resulted in a small reduction in the N stocks in the 0–20 cm layer in 2012 (Figure 1
). However, no differences were observed between ReR + F and ReM + F treatments in 2016 (Figure 2
) despite the higher harvest output in the ReR + F treatment. This can be attributed to the larger accumulation of N in the biomass in the ReM + F treatment, which was 33% higher when compared to the ReR + F treatment. This aligns with other long-term productivity studies that find that subsequent tree growth following stem only harvest tends generally to be higher than whole-tree harvest due to the maintenance of harvest residues on the soil [38
The available P stock was larger in the ReM + F treatment when compared with ReR + F treatment. This result could be caused by two main factors: (i) less ReM + F nutrient outputs by harvest and (ii) larger quantities of organic matter in ReM + F which reduces the fixation of P on the soil colloids, thus improving its availability. No differences were observed between treatments after 100 cm in depth. In a long-term productivity experiment conducted in North Wales, UK, by Walmsley et al. [9
], the alterations in soil characteristics and their effects were tested using different harvest treatments (whole tree harvest and bole-only harvest) on 23-year-old second rotation stands of Sitka spruce (Picea sitchensis
). The authors found that the whole-tree harvest treatment is responsible for the depletion of three to four times greater quantities of N, P, and K than the conventional bole-only harvest in the first rotation.
ACR + F and ReR + F treatments contained the largest stocks of S in the 40–100 cm layer in the soil, being statistically different when compared to others. This can be attributed to the lower concentration of soil organic C (SOC) of the previously mentioned treatments. Low organic matter (OM) in highly weathered soils results in a higher quantity of positive charges, especially in the B horizon [39
], with consequent retention of SO42−
in the soil.
Repeated removal of forest harvest residues is likely to reduce oxidizable C, available N, and total P stocks in the soil under Eucalyptus
plantations. The use of harvest residues as soil coverage can protect the soil against erosion, improve or maintain the SOC, and improve both the quantity and availability of nutrients stored in the soil [5
4.2. Soil Contribution to Nutrients Absorbed by Trees
The N and P losses as a result of the two harvest rotations (2004 and 2012) exceeded the inputs from fertilization and atmospheric deposition, resulting in a net loss of N and P from the site. In this period, the net N balance for ReM + F, ACR + F, ReR + F, and ACR − N treatments was −255, −516, −787, and −770 kg·ha−1
, respectively. With regards to P, there was also a negative net balance of −24, −48, −70, and −128 kg·ha−1
in the ReM + F, ACR + F, ReR + F, and ACR − P treatments, respectively. The net balance was positive only for S, with approximately +120 kg·ha−1
for all treatments with the exception of the ACR − N treatment, which contained a net balance of −128 kg·S·ha−1
). N inputs through biological fixation are unlikely due to the absence of N-fixing weeds. In addition, one should not expect inputs of N from rock weathering as the soil was highly weathered (Ferralsol). The difference between nutrient losses due to harvesting and nutrients supplied via fertilization and atmospheric deposition must be made up for with nutrients absorbed from soil pools.
From 2004 to 2012, soil N stocks declined in the 0–20 cm layer for all treatments (Figure 1
). The ReR + F treatment was the most affected among the treatments due to the elevated nutrient outputs by harvest. For the ACR − N treatment, this was due to the small quantity of N applied via fertilization. In the ReM + F treatment, the 0–20 cm layer was responsible for providing 65% of the difference in the balance of soil N. The 0–20 cm layer was responsible for providing 35% of the difference in net soil N balance for the ACR treatments and 24% for the ReR + F treatment (Figure 1
and Table 4
). The maintenance of harvest residues on the soil increases soil C and N pools—especially in labile fractions—thereby increasing the capacity of upper layers to provide N to plants [14
]. The remaining difference possibly came from deeper layers of the soil profile and factors such as biological fixation by symbiotic associations which were not examined in this study [40
]. Despite the large difference in the nutrient outputs between ReM + F and ReR + F treatments (more than 500 kg·ha-1
in the net balance), no differences in the N stocks between these treatments were found in the deeper layers of the soil (20–200 cm). More research is necessary in order to fully understand the additional sources of N in such highly weathered, tropical forests.
Little reduction was observed in the stocks of available P in the 0–20 layer (Figure 1
). The ACR − P treatment presented the highest reduction (2 kg·ha−1
), while the ReM + F treatment presented a slight increase. This happened even with the negative balance of the nutrient for all treatments evaluated. No reduction in soil P below 40 cm depth was observed in 2016 (Figure 2
). The small reduction in P content contradicts the highly negative net balance of this nutrient across treatments. P balance for all treatments was −60 kg·ha−1
on average. The ACR − P treatment had a deficit of −128 kg·ha−1
while the reduction was only 6 kg·ha−1
from 0–200 cm of soil depth. This discrepancy suggests that the Eucalyptus trees take up soil P hidden in fractions not extracted by the traditional resin method of analysis. Eucalyptus species can uptake organic and inorganic P fractions by phosphatase and exudation of low molecular mass acids [41
]. More studies are necessary to clarify the complete origin of P absorbed by trees.
Even with a positive net balance of S stocks in the soil (Table 4
), a small reduction was observed in the stocks of the nutrient when comparing the evaluations in 2004 and 2012. This is due to the elevated mobility of the element in the soil, especially in the upper layers [42
], along with its consequent migration to and accumulation in the 40–100 cm layer (Figure 2
) where it will adsorb to positive charges on soil surfaces exposed by losses of OM [39
4.3. Management Considerations
Despite the small effect of whole-tree harvest on the soil nutrient stocks described by many authors [4
], this and other studies emphasize the importance of the maintenance of forest residues on the soil, especially in tropical sites with low buffer capacity soils, high productivity, and short cycle plantations [14
]. In these sites, removal of forest residues can result in loss of wood productivity of up to 40% due to the low nutrient pools remaining in the soil [16
In the past several years, increasing demands for the use of forest residues for bioenergy purposes has grown in Brazil. Models for the utilization of these residues are based on research in temperate regions, where soils often have higher buffer capacities and higher organic C contents. This study shows that in tropical conditions the use of forest residues for bioenergy purposes should be carefully considered, taking into account the unique conditions of each site. On steep sites and/or those with low buffer capacity, all forest residues should be retained on the soil in order to avoid soil erosion [43
] and depletion of soil nutrient pools [14
]. In sites with favorable conditions for residue removal, preference should be given to the coarse residues due to their high caloric power and reduced nutrient concentration. With the removal of forest residues, the application of high rates of fertilizer is necessary in order to avoid productivity losses and to ensure the sustainability of the silviculture system.
The high negative net balance, especially for N and P, and the relative low reduction in the availability of these nutrients in the soil draws attention to two main points. First, more attention should be given to production sustainability, mainly in sites harvested in the whole-tree harvest system. Second, more studies should be implemented to better understand the contributions of organic and inorganic (mainly to P) fractions of low lability on the supply of nutrients to trees. Our results and others [29
] suggest that Eucalyptus trees can access P fractions not identified by traditional soil analysis methods. Regarding N concentrations, even with high harvest outputs (more than 500 kg·ha−1
in each crop rotation) no response to N application by fertilization was found in terms of wood productivity in Brazilian conditions [28
]. More research is essential to better understand the sources and the cycle of this nutrient within tropical soil-plant systems.