Simulated and measured soil organic carbon (SOC) stocks (0–20 cm) under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at Qiyang.

<p>Simulated and measured soil organic carbon (SOC) stocks (0–20 cm) under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at Qiyang.</p

Correlationship between simulated and observed crop (i.e.,□wheat and ▪corn) grain yield data under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.

<p>Correlationship between simulated and observed crop (i.e.,□wheat and ▪corn) grain yield data under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.</p

Statistics comparing simulated and measured SOC stocks at the three long-term experimental sites.

a<p>Note: <i>n-RMSE</i>, the normalized-root mean square err; <i>d</i>, the index of agreement; <i>R²</i>, the sample correlation coefficient.</p

Soil organic carbon in the (1) active, (2) slow, and (3) passive pool under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.

<p>Soil organic carbon in the (1) active, (2) slow, and (3) passive pool under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.</p

Land management practices for the long-term experimental sites during the different blocks and periods used in the CENTURY model simulations.

<p>Land management practices for the long-term experimental sites during the different blocks and periods used in the CENTURY model simulations.</p

Estimated carbon sequestration potential (0–20 cm) under long-term hNPKM and NPKS fertilization treatments.

<p>Estimated carbon sequestration potential (0–20 cm) under long-term hNPKM and NPKS fertilization treatments.</p

Simulated and measured soil organic carbon (SOC) stocks (0–20 cm) under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at Yangling.

<p>Simulated and measured soil organic carbon (SOC) stocks (0–20 cm) under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at Yangling.</p

Correlation between simulated SOC and measured SOC under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.

<p>Correlation between simulated SOC and measured SOC under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.</p

The pools and flows of carbon in the CENTURY model.

<p>The diagram showed the major factors which control the flows <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0095142#pone.0095142-AlisterK1" target="_blank">[29]</a>.</p

Average monthly precipitation, maximum (Tmax) and minimum (Tmin) temperatures from 1990 to 2010 at (A) Changping, (B) Yangling, and (C) Qiyang.

<p>Error bars are standard deviations for the mean values from 1990–2010.</p