Sampling locations from the tree row into the cropping alley.

<p>The crop like symbol refer to sample plot.</p

Multiple regression analysis of environmental factors and photosynthetic rates of soybean in different treatments in 2012.

<p>*, ** indicate significant at 5% and 1% levels, respectively.</p><p>Note: Y is the predicted value of NA, X₁ is PAR, X₂ is air temperature, X₃ is RH, X₄ is CO₂ concentration, and X₅ is soil water content.</p><p>Multiple regression analysis of environmental factors and photosynthetic rates of soybean in different treatments in 2012.</p

12 and 26-year-old trees (1997 and 2012, respectively) intercropped with soybean at the same study site.

<p>Note: DBH is the diameter at breast height of tree.</p><p>^a Data of 1997 adapted from Simpson [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0129467#pone.0129467.ref022" target="_blank">22</a>].</p><p>^b1997 data not measured.</p><p>12 and 26-year-old trees (1997 and 2012, respectively) intercropped with soybean at the same study site.</p

Within plot microclimate and soybean responses 2, 4, and 6 m from tree row to three 26-year-old hardwood intercropping systems and monocropped soybeans.

<p>Across all treatments (control, poplar, maple, walnut), values in each row followed by the same letter are not significantly different (Tukey’s HSD, P<0.05).</p><p>* Significant at P<0.10 (10%) level.</p><p>Within plot microclimate and soybean responses 2, 4, and 6 m from tree row to three 26-year-old hardwood intercropping systems and monocropped soybeans.</p

Soil microbial biomass of five natural secondary forest types.

<p>Soil (A) Cmic and (B) Nmic of each forest type. CP–Chinese pine, SO–Sharptooth oak, AP–Armand pine and WS–Wilson spruce. Values are means±SD and letters denote significant differences among forest types (<i>P</i> = 0.05).</p

Soil physical factors of five natural secondary forest types.

<p>BD, bulk density; WHC, water holding capacity. Values are means±SD and letters denote significant differences among forest types (<i>P</i> = 0.05).</p

PCA-ordination biplot (PC1 vs PC2) of soil samples from five forest types and 25 soil microbial and physico-chemical factors.

<p>BD–bulk density, WHC–water holding capacity, SOC–soil organic carbon, TN–total nitrogen, TP–total phosphorous, TK–total kalium, AP–available phosphorous, AK–available kalium, Cmic–microbial biomass carbon, BR–basal respiration, Nmic–microibal biomass nitrogen. I–Chinese pine, II–Sharptooth oak, III–Chinese pine+Sharptooth oak, IV–Armand pine, V–Wilson spruce.</p

Biomass-relating ratios of five natural secondary forest types.

<p>Soil (A) Cmic/Nmic, (B) Cmic/Corg and (C) Nmic/TN of each forest type. CP–Chinese pine, SO–Sharptooth oak, AP–Armand pine and WS–Wilson spruce. Values are means±SD and letters denote significant differences among forest types (<i>P</i> = 0.05).</p

Soil chemical factors of five natural secondary forest types.

<p>SOC, soil organic carbon; TN, total nitrogen; TP, total phosphorous; TK, total kalium; AP, available phosphorous; AK, available kalium. Values are means±SD and letters denote significant differences among forest types (P = 0.05).</p><p>SOC, soil organic carbon; TN, total nitrogen; TP, total phosphorous; TK, total kalium; AP, available phosphorous; AK, available kalium. Values are means±SD and letters denote significant differences among forest types (<i>P</i> = 0.05).</p

Soil enzyme activities of five natural secondary forest types.

<p>Soil (A) invertase, (B) urease, (C) acid phosphatase and (D) calatase activities of each forest type. CP–Chinese pine, SO–Sharptooth oak, AP–Armand pine and WS–Wilson spruce. Values are means±SD and letters denote significant differences among forest types (<i>P</i> = 0.05).</p