Leaf C:N ratio and nitrogen use efficiency

A series of model experiments with different fertilization treatments show that in the dynamic N scheme canopy mean leaf C:N ratio can quickly (within two years) adapt to enhanced soil mineral N conditions, while reduced N fertilization of 200 kg N ha⁻¹ yr⁻¹ will just be enough to sustain the level of soil mineral N concentration and leaf N concentration (Fig. 5.5). A FCN

value of 0.2 in the dynamic N scheme can represent most of the variability in leaf C:N ratio measured in the field, in both dry and rainy seasons, from different ages of palms and across site conditions of different fertilization treatments (data from Kotowska et al., 2016). When fertilization drops to 100 kg N ha⁻¹ yr⁻¹ and below, both soil mineral N and leaf N concentrations will decrease to the level below the initial conditions in the long run. However, 100 kg N ha⁻¹ yr⁻¹ fertilization can still meet the high yield demand of oil palm as shown in Fig. 5.6. At this level of fertilization, the NUE is above 100%, suggesting taking up of extra soil mineral N by the plant to compensate the deficiency in fertilizer supply. This matches the trend of depleting soil mineral N near the end of a rotation cycle (~25 years, Fig. 5.5). If the amount of fertilization keeps at the level of 100 kg N ha⁻¹ yr⁻¹, the total and average yield over the 25 years period is undistinguishable compared to the current practice of using much more fertilizer (400 kg N ha⁻¹ yr⁻¹) at the study site PTPN-VI (Fig. 5.6). Although the soil becomes less fertile within 25 years of cultivation, the purpose of oil palm fruit production is sufficiently met by the high NUE. Soil fertility could be restored or enhanced by appropriate N fertilization or using organic fertilizer as an alternative, either during the gap period before next rotation cycle or in the beginning of new plantation establishment when the young palms’ N demand is significantly lower.

Figure 5.5. Leaf C:N ratio (upper panel) by the default N scheme and the dynamic N scheme with different N fertilization treatments (400 kg N ha⁻¹ yr⁻¹ is the normal practice in PTPN-VI).

The FCN values set the maximum allowed fraction of departure of C:N ratios from the standard C:N in the dynamic N scheme (Table 5.1). The points and error bars are field measured leaf C:N ratio (mean and standard deviation) from different ages of palms (square for rainy season, triangle for dry season). Fertilization frequency is every 6 months. The tip of each zig-zag curve of the soil mineral N content (lower panel) marks a fertilization event.

Figure 5.6. Nitrogen use efficiency (NUE) and productivity in yield simulated by the default N scheme and the dynamic N scheme. NUE is defined by the ratio between N export with harvest and the amount of fertilizer N applied per year. Yield is the annual average during a full cultivation cycle (25 years).

On the contrary, further increasing fertilization to 800 kg N ha⁻¹ yr⁻¹ from the current practice, the yield could only slightly increase (8%). But the drop of NUE to 20% indicates a much lower efficiency of fertilizer utilization. The 80% excess fertilizer in the soil will potentially increase GHG emission including nitrous oxide (N2O), and/or leaching of nitrate, which is significant contributor to global warming and/or local and regional water quality problems (Brumme and Beese, 1992; Dowdell et al., 1979; Goulding et al., 2000).

Combining fertilization experiments by the model and field records from other smallholder plantations which apply much lower fertilizer (Chapter 2), it suggests that the current fertilization in PTPN-VI is overly applied. Using less than half of fertilizer (100 to 200 kg N ha⁻¹ yr⁻¹) would have been the most economical (with similarly high yield and lower fertilizer cost) and most sustainable (in regard of soil fertility and GHG emission) fertilization management.

Overall, the new dynamic N scheme allows to simulate N fertilization effects on the growth and yield as well as plant-soil N balance in the oil palm monoculture system. It can improve the adaptability of CLM-Palm and CLM4.5 to a wider range of soil nutrient conditions and fertilization practice. For the application of the dynamic N scheme on agricultural systems, the modeled trend of NUE together with yield variability with different levels of fertilization in the industrial oil palm plantation has comparable patterns with those observed in field fertilization experiments on winter wheat (Brentrup and Pallière, 2010) or other crops (e.g. rice and oat) or pasture (de Wit, 1992). In contrast, the CLM4.5 default N scheme with fixed C:N ratios does not fit well for modeling agricultural systems that are highly fertilized. The static N scheme is only able to simulate a limited range of fertilization effects under N shortage, relying heavily on the N downregulation mechanism on GPP which is a flawed approach due to its disconnectedness with the photosynthesis-stomatal conductance model (Bonan et al., 2012).

This scheme cannot simulate fertilization enhancement effects, which is, however, useful information for agricultural management and planning.

Beyond this, nutrient availability has been found to be the key regulator of net ecosystem production (NEP) and therefore C sequestration of forested landscape (Fernández-Martínez et al., 2014). It is critical to consider the change of nutrient availability induced by human activities (e.g. N deposition and fertilization) together with climate change factors (e.g. rising atmospheric CO2) when quantifying the trend of terrestrial C sequestration by both natural and managed ecosystems (De Vries and Posch, 2011; Luo et al., 2004). The ability to simulate realistic N cycle and its interaction with C cycle should be emphasized in Earth system models (Piao et al., 2013; Zaehle and Friend, 2010; Zaehle et al., 2010). This study shows that the

traditional strictly coupled C-N model with static C:N ratios should be improved with a relaxed and more realistic relationship between C and N cycling that can reflect the regulating effects of nutrient availability on both photosynthesis and heterotrophic respiration as well as C allocation pattern.