Abstract
Converting forest and wetland landscapes to agriculture has shown to result in a loss of organic matter, structure, and microbial diversity in the converted soil but recovery of post-agricultural soils remains poorly understood. Here we coupled landscape-scale surveys of soil 1) carbon and nitrogen levels, 2) aggregation, and 3) bacterial metagenomes to investigate soil recovery after 30 years in sites with soils ranging from well drained to poorly drained. Sites with no evidence of past agriculture (Reference) served as recovery endpoints. A secondary aim evaluated the role of nitrogen-fixing symbiosis, here associated with alder (Alnus incana) trees, in soil restoration. Soil carbon levels in restored sites (3.5%) were comparable to levels in a present-day farm (3.4%) but much lower than in Reference sites (>7.3%). The same trend occurred with soil nitrogen levels. Sites with alder trees had more acidic soil pH values. Alder trees promoted soil structure with macroaggregates being the largest fraction of bulk soil (75%). Natural abundance of stable nitrogen isotopes suggested extensive decay of organic matter within aggregates. Comparison of total reads from the soil metagenomes indicated the bacterial community in restored sites were more comparable to the present-day farm than Reference sites, except for a well-drained soil with alder. Dissimilarity among sites in terms of gene abundances in soil bacterial community occurred in carbon metabolism, membrane transport, and genetic repair pathways. Soil recovery in post-agricultural landscapes is slow when agriculture caused a large loss of soil organic matter, as is the case in our study, and when the soil bacterial community structure changed markedly, as it did in our study. However, fairly rapid recovery of soil structure, as we noted in our study, is promising, and now we need a better understanding of plant species that improve soil structure for restoration of both well-drained and poorly drained soils.
Highlights
Agriculture covers 38% of Earth’s land surface with much of it on land that naturally supports forests and wetlands
The research was conducted in central New York State
Given the experimental design with a large number of pairwise comparisons of sites (N 55 pairs of sites), as well as the number of ortholog sequences in the Kyoto Encyclopedia of Genes and Genomes (KEGG) database (>20,000), we only report the number of genes that were significantly enriched in some of the more relevant pairwise comparisons (Table 7)
Summary
Agriculture covers 38% of Earth’s land surface with much of it on land that naturally supports forests and wetlands. Converting forests and wetlands to agriculture often initiates a downward spiral in soil conditions (Kuzyakov and Zamanian, 2019). Agricultural production drops or fails altogether forcing abandonment. While many studies examine recovery of forest and wetland plant communities in post-agricultural landscapes (cf., Flinn and Vellend, 2005), a crucial uncertainty concerns how concomitant soils reclaim their natural conditions (Lal, 2015). Degraded soils experience to some degree: 1) loss of soil organic matter, 2) deterioration of soil structure, 3) scant or excessive levels of nutrients, and 4) altered microbial diversity and activity (Carter, 2002). The reclamation of each is still a matter of conjecture
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