Improving Productivity, Profitability and Sustainability of Degraded Grasslands Through Tree-Based Land Use Systems in the Philippines - Smallholder Tree Growing for Rural Development and Environmental Services - page 293

14.3.5.3 Changes in Soil Organic Nitrogen and Phosphorus

Predicted total soil organic nitrogen declined more slowly under the tree-based land
use systems (TIMPLUS, TCLUS, and TPLUS) than under the current practice of
the annual maize cropping (TCLUS) system (Fig. 14.5). The predicted soil organic
nitrogen was highest under the TPLUS system because of the nitrogen and organic
matter cycled through leaf litter and also because of the pruning during the first
year's rotation. Although there was a decline in soil organic nitrogen during tree
growth, it increased at a higher level after timber harvest, though not at the same
level as the initial soil nitrogen content. The soil total organic nitrogen slowly built
up after harvest due to the addition of organic matter from plant residues. Similar
pattern was observed under the IMPLUS system but soil nitrogen slowly declined
in a continuous and linear manner throughout the simulation period and at a much
slower rate compared to those under the FPLUS system. The average total soil
nitrogen under FPLUS system was 5.4 t ha −1 compared to 6.0 t ha −1 under IMPLUS;
5.9 t ha −1 under TIMPLUS, TCLUS and TCSFLUS systems; and 6.1 t ha −1 under the
TPLUS system (Appendix 3).
As in total soil nitrogen, the predicted total soil organic phosphorus showed a
downward trend over the simulation period (Fig. 14.5). The lowest decline of soil
phosphorus occurred under the TPLUS system while the greatest was under the
FPLUS system. The IMPLUS system sustained the soil phosphorus at a level similar
to the systems with the trees component. The average total soil phosphorus over 20

6.5

4.4

4.2

6.0

4.0

5.5

3.8

3.6

5.0

3.4

4.5

3.2

4.0 3.0

0 5 10 15 20 0 5 10 15 20

Year Year

IMPLUS FPLUS TIMPLUS

IMPLUS FPLUS TIMPLUS

TCLUS TCSFLUS TPLUS

TCLUS TCSFLUS TPLUS

Fig. 14.5 Predicted soil organic nitrogen and phosphorus (t/ha) over time of alternative land use
systems, Claveria, Misamis Oriental, The Philippines 4

4 The trends or the shapes of the curves for soil carbon, soil organic nitrogen and phosphorus were
similar probably because the quantities of these nutrients were calculated from the same initial
amount of soil. Another reason may be due to the model's limitation to capture actual soil nutrient
loss rather than relative to its original conditions before the simulation period.