Fig 1.
The photos of monocultural cassava farmland and the seasonal variations of precipitation and air temperature.
The photos are the cassava farmland (A) before harvesting in the rainy season and (B) after harvesting in the dry season. The forest visible in the background of photos is a protected forest (the photos were taken by A. Popradit, the first author). The research site is the frontline location where the farmland is encroaching into protected forest areas. (C) The monthly precipitation (bars) and the monthly averaged air temperature (closed cycles) during the research period (2016 to 2020). Vertical vars show ± 1 S.D.
Fig 2.
The properties of the forest soil and the farmland soil in 2016, 2018, and 2020.
(A) Kjeldahl nitrogen (the sum of organic N and ammonia N; g kg-1), (B) available phosphorus (P; mg kg-1), (C) exchangeable potassium (K; mg kg-1), (D) organic carbon (g kg-1), (E) pH, (F) electrical conductivity (EC; mS m-1), (G) cation exchange capacity (cmol(+) kg-1), (H) fraction of sand (%), (I) fraction of silt (%), (J) fraction of clay (%), (K) bulk density (Mg m-3), and (L) gravimetric moisture content (%). The boxplot shows the median (horizontal bar), interquartile range (box), 5th and 95th percentiles (whiskers; vertical bars), and outliers (black closed points). Different letters (a, b, c) show the significant differences among the years, as follows: ***: P < 0.001, **: P < 0.01, *: P < 0.05, and n.s.: P ≥ 0.05.
Table 1.
The statistical differences among the sampling years and forest or among the locations for each soil property, using repeated one-way ANOVA.
Table 2.
The results of multiple regression analysis for the cation exchange capacity.
Fig 3.
Relationships of the cation exchange capacity against (A) log10(organic carbon), (B) log10(the fraction of clay), and (C) pH. Open circles represent data in the farmland soils, and closed green circles represent data in the forest soils. ***: P < 0.001, *: P < 0.05.
Table 3.
The concentrations of herbicides (paraquat and glyphosate) in the stream water (upper, middle, and lower streams) and the farmland soil.