The ratio between nitrogen and phosphorus (N/P) in plant leaves has been widely used to assess the availability of nutrients. However, it is challenging to rapidly and accurately estimate the leaf N/P ratio, especially for mixed forest. In this study, we collected 301 samples from nine typical karst areas in Guangxi Province during the growing season of 2018 to 2020. We then utilized five models (partial least squares regression (PLSR), backpropagation neural network (BPNN), general regression neural network (GRNN), PLSR+BPNN, and PLSR+GRNN) to estimate the leaf N/P ratio of plants based on these samples. We also applied the fractional differentiation to extract additional information from the original spectra of each sample. The results showed that the average leaf N/P ratio of plants was 17.97. Plant growth was primarily limited by phosphorus in these karst areas. The sensitive spectra to estimate leaf N/P ratio had wavelengths ranging from 400–730 nm. The prediction capabilities of these five models can be ranked in descending order as PLSR+GRNN, PLSR+BPNN, PLSR, GRNN, and BPNN when considering both accuracy and robustness. The PLSR+GRNN model yielded high R2 and performance to deviation (RPD), and low root mean squared error (RMSE) with values of 0.91, 3.15, and 1.98, respectively, for the training test and 0.81, 2.25, and 2.46, respectively, for validation test. Compared with the PLSR model, both PLSR+BPNN and PLSR+GRNN models had higher accuracy and were more stable. Moreover, both PLSR+BPNN and PLSR+GRNN models overcame the issue of overfitting, which occurs when a single model is used to predict leaf N/P ratio. Therefore, both PLSR+BPNN and PLSR+GRNN models can be used to predict the leaf N/P ratio of plants in karst areas. Fractional differentiation is a promising spectral preprocessing technique that can improve the accuracy of models. We conclude that the leaf N/P ratio of mixed forest can be effectively estimated using combined models based on field spectroradiometer data in karst areas.
Gas exchange capacity of leaves is mainly restricted by the content of N
and P and environmental factors. However, the effects of interaction
between N and P and environmental factors on photosynthetic capacity in
subtropical tree species remain unclear. We measured the gas exchange
parameters (25℃ maximum carboxylation rate [Vcmax,25], and 25℃
maximum electron transport rates [Jmax,25]) and the chemical
properties of leaves (leaf N, leaf P and N:P) in 9 local dominant
species in the subtropical non-karst and karst regions of southwest
China. Environmental factors (temperature [Temp] and soil moisture
content [SMC]) of the study site were also monitored at the same
time. We found that P restriction is common in different research sites.
The results of the mixed linear model show that with the increase of
leaf N content of karst species, the sensitivity of Vcmax,25 to leaf P
increased significantly, and there was a significant interaction of N×P
(P < 0.001). Non-karst species tend to N×SMC interaction (P =
0.04). The difference in N×P interaction on gas exchange parameters
between non-karst and karst species might result from the decoupling
phenomenon of N and P caused by climate change. Factors such as N
sedimentation and soil P loss aggravate the N:P imbalance and lead to
the decoupling effect between N and P elements, and continuously weaken
the influence of N×P interaction on plant Vcmax and Jmax.
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