# Carbon, Water, and Light Use Efficiency Under Conservation Practice on Sloped Arable Land
## 3 Results
### Net Primary Productivity, Gross Primary Productivity, and Carbon Use Efficiency  
Biomass samples were collected during the study period at key stages of development for both crops. Belowground biomass, net primary productivity (NPP), gross primary productivity (GPP), and autotrophic respiration (AR) were calculated, as described in Section 2.4. To assess the effect of incorporating a cover crop into the rotation while minimizing operations and treatments along the contour, we estimated the average biomass accumulated in the rotations over a three-year period. The total biomass accumulated in the wheat-cover crop-maize rotation with minimum tillage was, on average, 37% (ANOVA, \(p\) = 0.0381) greater than the biomass accumulated in the standard wheat-maize rotation, implemented up and down the slope (Table 3). As a result of the conservation management approach, the average gross primary productivity (GPP) over the three-year period was 39% higher (6.59 \(\pm\) 0.35 kg C m\({}^{-2}\) year\({}^{-1}\)) compared with the conventional rotation (4.73 \(\pm\) 0.39 kg C m\({}^{-2}\) year\({}^{-1}\)), with the difference being statistically significant (ANOVA, \(p\) = 0.0072). Similar trends were observed for net primary productivity (NPP) and autotrophic respiration (AR), both of which exhibited statistically significant increases under the conservation treatment. GPP, NPP, and AR are graphically presented by rotation in Figure 2a-d. Moisture supply and nutrient storage, resulting from fertilization during erosion control practices, contribute to the increase in GPP and NPP, along with autotrophic respiration. Carbon use efficiency (CUE) is typically considered a dimensionless constant by most authors [46]; however, some studies suggest that it varies depending on the temperature and moisture regime [47]. In maize grown after a cover crop and along the contour, a decrease in carbon use efficiency (CUE) was observed, with values declining from 0.60 in 2021 to 0.55 in 2022 and 0.44 in 2023. In conventionally grown maize, CUE ranged from 0.59 to 0.52 over the same period (Table 4). Although the differences in CUE were not statistically significant, it is noteworthy that maize--the crop most affected by the summer \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Crop** & **Tillage** & **2021** & **2022** & **2023** \\ \hline Surface water runoff, m\({}^{3}\) ha\({}^{-1}\) & maize & CT & 821.7 & 305.8 & 136.8 \\  & & MT & 465.6 & 208.9 & 103.15 \\  & wheat & CT & 603.3 & 77.7 & 52.5 \\  & & MT & 375.9 & 62.2 & 38.1 \\ \hline Soil loss t ha\({}^{-1}\) & maize & CT & 10.9 & 3.1 & 2.4 \\  & & MT & 4.4 & 1.9 & 0.5 \\  & wheat & CT & 7.7 & 0.9 & 0.6 \\  & & MT & 2.7 & 0.4 & 0.2 \\ \hline \hline \end{tabular} ANOVA: surface water runoff; wheat significance \(p\) = 0.00013, maize significance \(p\) = 0.00020; eroded soil wheat significance \(p\) = 0.0000; maize \(p\) = 0.0000.  
\end{table} 
Table 2: Total volume of surface water runoff (m\({}^{3}\) ha\({}^{-1}\)) and amount of eroded soil (t ha\({}^{-1}\)) annually, for the period 2021–2023. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Rotation** & **Aboveground** & **Belowground** & **NPP** & **GPP** & **AR** \\ \hline MT, wheat–cover crop–maize & 6.17 \(\pm\) 0.65 & 2.53 \(\pm\) 0.30 & 3.62 \(\pm\) 0.38 & 6.59 \(\pm\) 0.35 & 2.97 \(\pm\) 0.22 \\ CT, wheat–maize & 4.50 \(\pm\) 0.42 & 1.84 \(\pm\) 0.23 & 2.63 \(\pm\) 0.25 & 4.73 \(\pm\) 0.39 & 2.10 \(\pm\) 0.18 \\ \hline \hline \end{tabular} ANOVA: aboveground, belowground biomass, and NPP, significance \(p\) = 0.0381; GPP significance \(p\) = 0.0072; AR significance \(p\) = 0.0115.  
\end{table} 
Table 3: Average accumulated values for the period 2021–2023: measured aboveground biomass (kg m\({}^{-2}\)), belowground biomass (kg m\({}^{-2}\)), and simulated net primary productivity (NPP), gross primary productivity (GPP), and autotrophic respiration (AR) (all in kg m\({}^{-2}\)) across crop rotations. 2023--exhibited a decline in CUE under both management treatments.