Soil recovery of alluvial gold mine spoils in the Peruvian Amazon using Stylosanthes guianensis, a promising cover crop

Abstract

The Amazon is an important reservoir of biodiversity and carbon but it is under pressure by multiple threats such as artisanal and small‐scale gold mining (ASGM). In Peru ASGM has degraded 90,000 ha of old‐growth forest since the eighties, leaving vast areas as wastelands. As most ASGM in the region is illegal, efforts to recover degraded areas have been scant. Here we assessed the potential of Stylosanthes guianensis to recover soil health as a first step in the restoration of gold mine spoils in a Native community and a mining concession in Madre de Dios, Peru. We evaluated plant growth and analyzed changes in physical, chemical, and biological soil parameters. After 470 days from sowing, the average plant height was 46.7 cm with a survival rate >50% and yields of 23.9 t ha−1 and 450 kg ha−1 of dry biomass and nitrogen, respectively. Multiple soil parameters increased significantly, including cationic exchange capacity (3.3 to 4.0 cmol [+] kg−1), soil organic matter (0.03% to 0.39%), soil respiration (0.02 to 0.06 mg CO2 g−1 d−1) and biomass (0.03 to 0.15 mg C g−1). Soil macrofauna increased from 2 to 11 taxonomic groups, including ants, considered as soil engineers. Furthermore, S. guianensis increased soil carbon sequestration of impacted areas from 0.004 t C ha−1 by more than 1650%, up to 0.07 t C ha−1. These promising findings clearly illustrate S. guianensis potential to kick‐start natural succession of Amazonian forests after degradation by ASGM and hence help to achieve the Sustainable Development Goals.

Global patterns and edaphic-climatic controls of soil carbon decomposition kinetics predicted from incubation experiments

“…Precisely, soil in humid tropical forests with slightly lower clay content is less probable to encounter physical protection of mineral-associated organic matter, resulting in higher k ref of the fast and slow pools 58 . However, the significantly lower pH (e.g., <5 units) constrains fungal decomposition of chemically-recalcitrant SOM 84 . In the Australian desert areas, the response of k ref to the explanatory factors is contrary to that of the Amazon Rainforest, which is probably because the relatively high pH (e.g., 7−9 units) limits bacterial activities but maximizes fungal energy efficiency 78 , 85 .…”

Resilience of Aboveground Biomass of Secondary Forests Following the Abandonment of Gold Mining Activity in the Southeastern Peruvian Amazon

“…These results indicate that heavy machinery mining may have a considerable negative effect on species diversity [21], because suction pumping mining forest stands harbor on average approximately two times more species than heavy machinery stands. Given that species diversity usually starts near zero with no woody plants, due to the elimination of vegetation cover [39,41,42] and severe soil degradation [20,43,44,54] by goldmining, our results highlight the lower impact of suction pumping mining on landscape structure, which promotes an increase over time as new species become established [79], and facilitates the faster recovery across the landscape [80]. The lower species diversity after mining abandonment may be explained by a higher dominance (IVI > 10, Table 3) or monodominance [30,95] of pioneer species in 0-5-year-old stands, due to higher tolerance of certain species to unfavorable soil conditions [95] generated by goldmining activities in the study area.…”

“…In the Southeastern Peruvian Amazon (Madre de Dios), studies on biomass estimates in natural regeneration after abandonment by mining activities are scarce. Most studies have focused on assessing the species diversity, floristic attributes, species composition [20,40,41], reforestation/rehabilitation experiments of these degraded ecosystems [42][43][44][45][46], and monitoring forest loss due to goldmining [19,26]. These studies showed that the degree of resilience of tree communities after abandonment by mining activities depends on the level of landscape fragmentation [41], stand age [20], and distance from the forest edge [41].…”

“…These studies showed that the degree of resilience of tree communities after abandonment by mining activities depends on the level of landscape fragmentation [41], stand age [20], and distance from the forest edge [41]. Meanwhile, the experiments in these areas demonstrated the rapid acclimatization of native tree species [44,46,47] and cover plants [43] to these severely degraded environments.…”

Tropical surface gold mining: A review of ecological impacts and restoration strategies

“…Plantings of legumes can improve mine substrates by fixing nitrogen into the soil, and may also serve as a sustained source of organic matter by providing leaf litter (Peláez et al, 2013; Schimann et al, 2012; Thomas, 2014). Established plants also improve soil quality by increasing soil pH, soil organic carbon, available phosphorus, exchangeable potassium (Peláez et al, 2013; Tetteh, Logah, et al, 2015), cation exchange capacity, microbial activity (Couic et al, 2018; Schimann et al, 2007; Velásquez Ramírez et al, 2021) and aggregate stability (Peláez et al, 2013). Plantings of leguminous trees can also increase soil biomass and microbial activity (dos Santos et al, 2013), although their impact may be higher in clay‐based soils than in sandier ones (Schimann et al, 2007).…”